(covers information from several alternate timelines)
The Vulcan mind meld or mind probe was a telepathic link between two individuals. It allowed for an intimate exchange of thoughts, thus in essence enabling the participants to become one mind, sharing consciousness in a kind of gestalt. (TOS: "Dagger of the Mind"; TNG: "Sarek"; et al.) Normally, it was employed only by Vulcans. It was a deeply personal thing, part of the private life, and generally not used on aliens. (TOS: "Dagger of the Mind") However, cases were known where the mind meld was initiated between a Vulcan and a non-Vulcan. (VOY: "Meld"; TNG: "Sarek")
Physical contact was not strictly required, but could enhance the effectiveness of the Vulcan's telepathic abilities. (TOS: "The Devil in the Dark") When physical contact was used in a mind meld with a humanoid, the initiator placed the tips of his fingers at key locations on the head of the other participant. (TOS: "Mirror, Mirror", et al.)
A mind meld could also be used by its initiator to probe another person's mind, while the melder shielded his or her own mind from being read by the other participant. (Star Trek VI: The Undiscovered Country)
Some species, including Cardassians, could resist the technique, shielding their thoughts from the attempted probe. Cardassian Gul Dukat resisted a forcible attempt by Vulcan Maquis member Sakonna to mind meld with him during an interrogation Sakonna performed on behalf of the Maquis. Dukat explained this ability to resist as "simply a matter of discipline." (DS9: "The Maquis, Part II")
A mind meld could even be used to transfer the entire personality or "soul" (known to Vulcans as the katra) of an individual into another body. (Star Trek II: The Wrath of Khan, Star Trek III: The Search for Spock) A special, appropriately prepared receptacle, such as a katric ark, could also be used. (Star Trek III: The Search for Spock) Vulcans (and Trill, through the zhian'tara ritual) did this without aid, whereas a number of people and species were able to achieve similar feats with technology, such as the people of Sargon's planet, Janice Lester, Dr. Ira Graves, Rao Vantika, and Tieran. (Star Trek III: The Search for Spock; TOS: "Return to Tomorrow", "Turnabout Intruder"; TNG: "The Schizoid Man"; DS9: "The Passenger", "Facets"; VOY: "Warlord")
Melding carried some risk. It could be physically debilitating for both parties and involved pressure changes which could potentially aggravate existing conditions. The melding resulted in some loss of identity, and could be difficult to break, especially when the subject's mind was powerful or dynamic. Aftereffects could be treated with the drug lexorin. (TOS: "Dagger of the Mind", "The Devil in the Dark"; VOY: "Ex Post Facto", "Meld", "Flashback"; Star Trek III: The Search for Spock)
During the 22nd century, mind melding was believed to be an ability only a minority of Vulcans were born to. Because of the apparent intimacy of melding, Vulcans during this era considered it a deviant practice defiant to the ancestral teachings of their society. As a result, other Vulcans considered those who were natural "melders" to be outcasts. (ENT: "Stigma")
An improperly trained melder who initiated a mind meld could cause a degenerative neurological disorder known as Pa'nar Syndrome to develop in the meld recipient. If left untreated, the disorder was fatal. Vulcan medicine of the mid-22nd century held that there was no cure; however, the disorder was known in Surak's time, as was its cure: a corrective meld performed by an experienced melder. It is likely that the ignorance of this cure was due to the widespread prejudice against melders, a prejudice that was not shared by a group of Vulcans called the Syrrannites. (ENT: "Fusion", "Stigma", "Kir'Shara")
Another side-effect was the transfer of emotion. When Spock melded with James T. Kirk of the alternate reality, both seemed emotionally affected by the experience and Spock apologized, explaining that emotional transference was a side-effect of the melding process. (Star Trek) Indeed, melding created a strange sense of euphoria in the participants. (TOS: "Dagger of the Mind")
In 2151, the v'tosh ka'tur Tolaris initiated a mind meld with Sub-Commander T'Pol against her will. Since Tolaris had not been properly trained in the use of his mental abilities, this meld caused T'Pol to develop Pa'nar Syndrome. (ENT: "Fusion", "Stigma")
In 2154, Vulcan Ambassador Soval, defying the taboo of his day, melded with a comatose Corporal Askwith to learn who had bombed the United Earth Embassy on Vulcan, deciding it was worth the risk upon discovering that evidence implicating the Syrrannites was fabricated. For this offense, Administrator V'Las (who was actually behind the bombing) relieved Soval of his post. (ENT: "The Forge")
In November of 2154, T'Pol initiated her first mind meld on the starship Enterprise NX-01. In an attempt to ultimately discover the location of Doctor Phlox, T'Pol melded with Ensign Hoshi Sato. (ENT: "Affliction")
In January of 2155 of the mirror universe, T'Pol, via a mind meld, implanted a telepathic "suggestion" into the mind of Commander Charles Tucker. Under this influence, Tucker sabotaged the power grid of the ISS Enterprise in order to disable a Suliban cloaking device. After doing this, T'Pol melded with Tucker again, erasing his memories of his actions. (ENT: "In a Mirror, Darkly")
In the alternate reality, Spock Prime melded with young Kirk in 2258 to explain how he and Nero came from the future, showing him through images the events that led to the creation of the alternate reality itself. During the meld, some of Spock's own emotions transferred over to Kirk about the Destruction of Vulcan when he showed Kirk his perspective of the event. Furthermore, Spock of the alternate reality mind-melded with a Romulan to find out where Captain Pike was being held. (Star Trek)
In the alternate reality, Spock melded with Christopher Pike in 2259 during his death, caused by Khan Noonien Singh. Later that year, Spock also melded with Khan during a fight, in an attempt to stop him. (Star Trek Into Darkness)
In 2266, on stardate 2715, Spock melded with Simon Van Gelder, then suffering from generalized synaptic damage, to learn whether allegations Van Gelder had made about Dr. Tristan Adams were true, or delusions. Spock warned Van Gelder that the pressure changes involved in the meld could be dangerous. (TOS: "Dagger of the Mind")
The same year, on stardate 3192, Spock used what might have been a variation of a mind meld to convince a guard on Eminiar VII to open the door to a room where a landing party was imprisoned, facilitating their escape. (TOS: "A Taste of Armageddon")
Also in 2267, McCoy became the victim of an unwilling mind meld, when the mirror universe Spock forced information from him about the nature of a transporter accident that had brought four USS Enterprise officers into the mirror universe. (TOS: "Mirror, Mirror")
On stardate 4657.5, Spock attempted a meld with Kelinda, a Kelvan, through a few feet of solid rock. Although the attempt was thwarted by Kelinda with prejudice, Spock was able to get an impression of the Kelvan's true non-humanoid form. (TOS: "By Any Other Name")
In 2268, on stardate 4385.3, Spock melded with Kirk, McCoy, and Montgomery Scott to ensure they believed that bullets fired at them from Virgil Earp, Morgan Earp, Wyatt Earp, and Doc Holliday (at the OK Corral) were unreal and, therefore, unable to harm them. (TOS: "Spectre of the Gun")
The same year, Spock melded with Kirk, who was suffering from retrograde amnesia, consequential to the misuse of a Preserver artifact. Although Spock had some difficulty emerging from this meld, he was able to successfully restore Kirk's memory. (TOS: "The Paradise Syndrome")
Later in 2268, Spock was driven mad by the sight of Medusan Ambassador Kollos. Miranda Jones, a Human telepath trained on Vulcan, used a meld to restore his sanity by making him forget what he'd seen. (TOS: "Is There in Truth No Beauty?")
In the 2270s, when the Enterprise was inside V'ger, Commander Spock entered a section of V'ger where it stored 3D images of objects that it encountered through its travels. He entered this area via thruster suit and subsequently encountered a large representation of Lieutenant Ilia with a pulsing sensor on her neck. Believing it to have some special meaning, he used a mind meld on it. Spock was overwhelmed by the information from the mind meld, and was flung back unconscious towards the Enterprise. (Star Trek: The Motion Picture)
In 2285, Spock used a mind meld to transfer his katra to McCoy before sacrificing himself to restore warp power during the Battle of the Mutara Nebula. (Star Trek II: The Wrath of Khan) Upon the Enterprise's return to Earth, Sarek mind-melded with Kirk to discover the fate of Spock's katra. (Star Trek III: The Search for Spock)
After time-traveling to 1986, Spock mind melded with humpback whales George and Gracie to inform them that his crew planned to bring the whales back with them to the year 2286, to answer a mysterious probe that threatened Earth; Spock reasoned that they had to confirm that the whales were willing to help them or they would be no better than those who made the whales extinct in the first place. (Star Trek IV: The Voyage Home)
In 2368, shortly following the death of Sarek, Spock entered into a mind meld with Captain Picard to touch the thoughts that Sarek had left before he died, Sarek and Spock having never melded while Sarek was alive. (TNG: "Unification II")
In 2370, Sakonna, a Vulcan member of the Maquis, attempted a mind meld on Gul Dukat in an attempt to learn the location of illegal Cardassian weapons in the Demilitarized Zone, but this mind meld failed due to Dukat's mental discipline allowing him to resist her efforts to access his memories. (DS9: "The Maquis, Part II")
In 2371, Tuvok melded with crewmate Lieutenant Tom Paris after Paris was accused of having murdered Banean scientist Doctor Tolen Ren. The meld allowed Tuvok to prove that Paris had been framed in an elaborate plot to provide the Numiri, who were enemies of the Baneans, with critical information, when his analysis of Paris' memory of the incident revealed crucial anomalies that proved Paris couldn't have committed the murder. (VOY: "Ex Post Facto")
In 2372, Tuvok melded with sociopath Betazoid Lon Suder, in an attempt to gain a better understanding of Suder's violent impulses. While the meld provided Suder with a measure of emotional control, it also temporarily released Tuvok's more violent and primal urges. (VOY: "Meld")
In 2373, Tuvok melded with his close friend Kathryn Janeway to discover the root of a supposed memory he had of a girl falling to her death from a precipice. For this meld, chairs were specially set up. It was later discovered that Tuvok had a memory virus in his brain, unwittingly transmitted to him in 2293 by Dimitri Valtane. The virus was destroyed with thoron radiation. (VOY: "Flashback")
Later that year, he melded with a Mari named Guill in an investigation to prove B'Elanna Torres' innocence in provoking a death in a telepathic society. Guill was from a society where violent thoughts were illegal, though he was a collector and trader of such thoughts. Tuvok proceeded with the meld under the pretense that only images would be exchanged, but he also grabbed and choked Guill in the meld to get him to submit and confess to his role in the death. (VOY: "Random Thoughts")
In 2377, Tuvok performed an extended technique of the mind meld, the "Bridging of Minds", on Captain Janeway and Seven of Nine, forming a bond between the two individuals, who were otherwise unable to telepathically link. (VOY: "Unimatrix Zero")
Background information Edit
Prior to the invention of the Vulcan mind meld, the scene it was first used in was to have been a lengthy interrogation of Simon Van Gelder by Captain Kirk. (I Am Spock, hardback ed., p. 60) When Robert Justman contacted Gene Roddenberry with a memo about the revised story outline of "Dagger of the Mind" (the memo was dated 4 April 1966), Justman remarked, "I feel like it would be best for us if we did not establish anything like a truth beam when Kirk attempts to interrogate Van Gelder. Perhaps the ship's doctor could have administered some sort of a sedative which have certain properties which would enable Kirk to get deeper into the heart of what is troubling Van Gelder."
Leonard Nimoy cited Gene Roddenberry as the inventor of the Vulcan mind meld. (Star Trek: The Real Story; I Am Spock, hardback ed., pp. 59 & 60) According to Nimoy, Roddenberry devised the idea in an attempt to make the scene it was first used in more dramatic. (I Am Spock, hardback ed., p. 60) In his reference book Gene Roddenberry: The Myth and the Man Behind Star Trek (p. 81), writer Joel Engel credited Shimon Wincelberg, who wrote TOS: "Dagger of the Mind", with the concept, rather than Roddenberry. The mind meld was thought up by the staff writers of Star Trek: The Original Series by the time they had fully devised the neural neutralizer (which is also established in "Dagger of the Mind"). (The Star Trek Compendium, 4th ed., p. 41)
Thereafter in the episode's development, Spock used a hypnosis machine to retrieve the much-needed information from Simon Van Gelder's mind. (Beyond the Final Frontier, p. 14) The method Spock employed was referred to as hypnotism in another memo from Bob Justman to Gene Roddenberry (this one dated 28 June 1966).
In the final draft script of "Dagger of the Mind", the mind meld wasn't referred to as an ancient technique, though it is described as that in the final version of the episode (in a log entry which wasn't at all in the final draft script). The teleplay's description of the mind meld between Spock and Van Gelder was as follows; "[Spock] places his hands at each side of the junction of neck and shoulder, his thumbs pressing deeply into Van Gelder's torso. Spock seems to be feeling with his fingers for [blood] vessels and nerve endings. Van Gelder groans, writhes a bit... Spock seems to be finding the pressure points he wants. He is analyzing what he feels, something of an expert safe-cracker feeling the action of mechanisms and tumblers through his fingers. Finally he's ready." In an ultimately abandoned scene from the same script, this meld was established as causing Simon Van Gelder to lose consciousness and nearly die, and Spock to become exhausted "at the emotion he has put into this."
The invention of the mind meld had the advantage of providing Spock actor Leonard Nimoy with a way to emphasize the importance of touch to Vulcans, which he was making a concerted effort to do. (I Am Spock, hardback ed., p. 60) Nimoy described the mind meld as "a wonderful creative idea" that Roddenberry devised when he was "at his best." (Star Trek: The Real Story) The actor further commented, "It was a far more dramatic way to extract information than a lot of questions, and it became a popular device for the show's writers [....] I applauded the concept, not only for the drama but because it gave me an opportunity to step outside the character from time to time." (I Am Spock, hardback ed., p. 61)
In the final draft script of ENT: "Vox Sola" but not in the episode itself, the Vulcan mind meld was mentioned by Captain Archer. He likened it to a predicament he had just been saved from, in which becoming caught along with other Human officers in a symbiotic lifeform's web-like tendrils had enabled the Humans to read each other's minds.
Likewise, in the final draft script of ENT: "Rajiin", mind melds were again mentioned in dialogue which didn't make it into the final edit of the episode. In that case, they were said by T'Pol to be dissimilar to invasive telepathy carried out by Rajiin, as mind melds were less "physically invasive" than the form of telepathy used by Rajiin and were more about reading minds than her telepathy was.
In the novelization of The Undiscovered Country, a slightly different take on the forced mind meld between Spock and Valeris was featured. As Spock probed her mind, he stopped just short of breaking her will and gave her the choice to willingly help him. A choked sob by Valeris was an expression of her gratitude and grief at being given the choice. Nevertheless, the scene deeply distressed Spock and shocked the crew.
In the novel Memory Prime, the mind meld was used as a form of combat between Spock and a Romulan assassin.
In the William Shatner novel The Return, Spock performed a meld with Picard and Kirk in an attempt to free Kirk from a brainwashing program that had been implanted in him by the Borg/Romulan Alliance; at the time, Spock said that such a thing had never been done, but it is possible that the Bridging wasn't well-known among Vulcans.