A constructed culture.
Concultures are associated with conlangs: this is my exotic/futuristic/imaginary language, and these are my exotic/futuristic/imaginary people who speak it. Every language needs speakers, and concultures are the speakers of conlangs.
The most well-known conculture would be J.R.R. Tolkien's Middle-Earth. Tolkien is known to have said he created Middle-Earth as a place where Quenya and his other languages could have something to attach to.
Also, the name of a mailing list where concultures are discussed.