Symbiotic lifeform

From Memory Alpha, the free Star Trek reference

A symbiotic lifeform is an organism which closely shares its existence with another species, usually for mutual benefit. A symbiotic lifeform may even merge with its host or symbiont.

Symbiotic lifeforms include:

• An unnamed host organism which managed to gain access to the NX class starship Enterprise in 2152. (ENT: "Vox Sola")

• The magnetic organism found aboard the Insectoid pod ship in orbit of Questar M-17. (TAS: "Beyond the Farthest Star")

• The Ceti eel, indigenous to Ceti Alpha VI. (Star Trek II: The Wrath of Khan)

• The Trill and their symbionts. (TNG: "The Host")

• Neural parasite aliens attempted to infiltrate Starfleet Command in 2364. (TNG: "Conspiracy")

• In 2372, the crew of the USS Voyager encountered a species of orchid that has symbiotic properties. It used its lysosomal enzymes to merge itself during transport at the microcellular level with Lieutenant Tuvok and Neelix, to create Tuvix. (VOY: "Tuvix")