A fictional language is separated from an artlang (language constructed for beauty or fun) by both purpose and relative completion: a fictional language generally has the least amount of grammar and vocabulary possible, and it is made usually for a novel or movie.
Others have developed languages in detail for their own sake, such as the languages of Middle-earth of J. R. R. Tolkien, Star Trek's Klingon and the languages in Star Wars.
See also: