
Tora Ziyal, a female Bajoran / Cardassian hybrid

B'Elanna Torres, hybrid split into Klingon and Human halves
A hybrid is the end result of mixing two or more different species in the biological process of reproduction.
Because of the common genetic ancestry of most of the species of the Milky Way Galaxy by way of the ancient humanoids, many species are able to interbreed with or without the help of genetic technology. In fact, Humans and Vulcans are quite similar. (TNG: "The Chase"; ENT: "Demons")
There can be unexpected consequences to interspecific relations and as such, it was a researched topic. Starfleet Academy taught Interspecies Protocol and the Ktarian Doctor Mizan was an expert in interspecies mating rituals, known for his "empirical research." (TNG: "Timescape"; VOY: "The Disease") One should note that there can be certain difficulties involved in reproducing between species, even humanoid, for anatomical reasons; for instance, not all species keep their genitals in the same place (Star Trek VI: The Undiscovered Country), or incompatible number of required genitals (DIS: "Choose Your Pain"), or the embryo will not survive to term due to genetic or physical reasons, such as life-threatening birth defects or dangers to the mother during birthing which may require caesarean section or fetal transport to save one or both. (VOY: "Deadlock")
In some cultures, children who are born as a result of interspecific relations are rejected by the society of both races. For example, Bajoran / Cardassian hybrids were often rejected by both Bajorans and Cardassians. (DS9: "Indiscretion", "Return to Grace") Klingons referred to hybrids derogatorily as "mongrels". (VOY: "Barge of the Dead", "Prophecy")
Examples of interspecific reproduction
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Betazoid
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- Betazoid / Human (Deanna Troi, Kestra Troi, Devinoni Ral, Anita Pierce, Walter Pierce)
- Betazoid / Tavnian (Lwaxana Troi's and Jeyal's son) (DS9: "The Muse")
Cardassian
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- Cardassian / Bajoran (Mika and Dukat's child, and Tora Ziyal) (DS9: "Covenant")
- Cardassian / Kazon (Seska and Culluh's child) (VOY: "Basics, Part I", "Basics, Part II")
Human
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- Human / Angel I native (the crewmembers of the Federation freighter Odin were mentioned to have fathered children with the Angel I inhabitants) (TNG: "Angel One")
- Human / Boraalan (Nikolai Rozhenko and Dobara's child) (TNG: "Homeward")
- Human / Betazoid (Deanna Troi, Kestra Troi, Devinoni Ral, Anita Pierce, Walter Pierce)
- Human / Cardassian (implied above)
- Human / Daliwakan (The Gigolo) (VOY: "The Cloud")
- Human / Denobulan (Denobulan child on future Enterprise) (ENT: "E²")
- Human / Ikaaran / 2 unknown species (Karyn Archer) (ENT: "E²")
- Human / Klingon (K'Ehleyr (medical help was needed (TNG: "The Emissary"), B'Elanna Torres, Miral Paris, Alexander Rozhenko)
- Human / Ktarian (Naomi Wildman, Sabrina) (VOY: "Deadlock", VOY: "Endgame")
- Human / Napean (Daniel Kwan) (TNG: "Eye of the Beholder")
- Human / Ocampa (Linnis Paris, Andrew Kim) (VOY: "Before and After")
- Human / Romulan (Sela, Simon Tarses and brother)
- Human / Skagaran (Bethany) (ENT: "North Star")
- Human / Terrellian (Human corpse) (ENT: "Future Tense")
- Human / Trill (Yedrin Dax, Lisa, several others) (DS9: "Children of Time")
- Human / Vulcan (Spock, Elizabeth, Lorian, Spock (alternate reality), Spock (mirror))
- Human / Xyrillian (Trip Tucker's pregnancy; it should be noted, however, that he was more of a carrier than an actual, biological parent) (ENT: "Unexpected")
- Q has implied that a Human / Q hybrid is possible (TNG: "True Q"; VOY: "The Q and the Grey") He also stated that the Q could breed with, or as, any species because of their godlike abilities.(citation needed • edit)
Klingon
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- Klingon / (Betazoid / Human) (Shannara and Eric-Christopher, alternate timeline children of Worf and Deanna Troi) (TNG: "Parallels")
- Klingon / Human (K'Ehleyr (medical help was needed), B'Elanna Torres, Miral Paris, Alexander Rozhenko)
- Klingon / Trill (not possible without medical help; individuals never appeared, as Jadzia Dax was murdered soon after confirmation that such a pregnancy was possible) (DS9: "Tears of the Prophets")
- Klingon / Romulan (Ba'el and several other prisoners in the Carraya IV colony) (TNG: "Birthright, Part I")
Ocampa
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- Ocampa / Human (Linnis Paris, Andrew Kim) (VOY: "Before and After")
- Ocampa / Talaxian (although they decided not to, Kes and Neelix seemed to imply it was possible for them to have a baby) (VOY: "Elogium")
Romulan
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- Romulan / Human (Sela, Simon Tarses)
- Romulan / Klingon (Ba'el and several other prisoners in the Carraya IV colony) (TNG: "Birthright, Part I")
Talaxian
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- Talaxian / Mylean (Neelix's great-grandfather was Mylean) (VOY: "Scientific Method")
- Talaxian / Mylean / Vulcan (Tuvix, was caused by a transporter malfunction with hybridizing flowers) (VOY: "Tuvix")
- Talaxian / Ocampa (implied above)
Trill
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- Trill / Human (mentioned above)
- Trill / Klaestron (Benjamin Sisko briefly assumed that Ilon Tandro was the son of Curzon Dax and Enina Tandro) (DS9: "Dax")
- Trill / Klingon (referenced above)
Vulcan
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- Vulcan / Human (Spock, Elizabeth, Lorian, Spock (alternate reality), Spock (mirror))
- Vulcan / Talaxian (seen above)