A coalescent organism was any organism (usually microscopic in size) capable of mimicking another organism down to a cellular level upon physical contact, usually resulting in the death of the host as a result of being absorbed by the organism. Not only did such creatures absorb and mimic the host's DNA, they could also absorb memory engrams from the host.
When Kahless the Unforgettable was reported to have returned at Boreth in 2369, Beverly Crusher speculated, amongst other scientific explanations, that he might be a coalescent organism. (TNG: "Rightful Heir")
Few such organisms are known to exist due to their need to frequently absorb a subject (and therefore change form) in order to survive (with smaller coalescent organisms, this is only a matter of seconds). A substantially larger coalescent organism had killed and absorbed Starfleet officer Keith Rocha in 2369, while he was on a deep space mission shortly before he was to transfer to Relay Station 47.
In Rocha's form, the creature transferred to Relay Station 47 and was able to assume his form for several weeks until it needed to absorb another organism. The only other two compatible organisms aboard Relay Station 47 were Aquiel Uhnari and her pet dog, Maura; the creature, still in the form of Rocha, attacked Uhnari, beginning the process of coalescence causing Uhnari to lose her short term memory.
Uhnari was able to escape the creature and injure it before fleeing the station via a shuttlecraft, leaving the creature to coalesce with Maura, which would later attack and attempt to absorb Lt. Cmdr. Geordi La Forge. Geordi was forced to destroy the creature in self-defense. (TNG: "Aquiel")
The coalescent organism has been an early application of CGI techniques in the television franchise. Visual Effects Supervisor for the episode, Ronald B. Moore, commented on its application, "Sometimes, things really work good and you are very happy with the way they came out, and sometimes you are not. And in "Aquiel" there were things in it that I was very, very happy with, and things that I wasn't. One of them was the monster. CGI was kind of coming into prominence, it was still very early. We had done stuff earlier in some of the first couple of seasons. "Aquiel" we got into and it just became a real time crunch, early CGI just took forever. I finally got this stuff, the CGI stuff that I composited in the bay [remark: Moore is referring to the post-production editing bay at Digital Magic], kind of at the end of our schedule, had no time, and I put it together. And there were a lot of funny names given to that monster when it came up, "Mr. Peanut" was probably the kindest one that I had. But, you know, it wasn't terrible, it was just for us it was something that we wished would have been a lot more better, with other words, the CBB could be better." (TNG Season 6 DVD special feature "Select Historical Data Year Six: To CGI or Not to CGI", disc 7) The "CBB", Moore referred to was an industry term for rendering and animation, which he had wanted to finetune. (Star Trek: The Next Generation Companion, p. 233)