These are the rules I go with,
collections of art that serve a game or a film production - be it a practical or an imaginary one - belong to concept art. (However, the backgrounds, storyboards, and vis dev for completed short films can be found under film.)
sequential or panel-based pieces are called comics.
observational work, practices, prompted series go to sketchbook.
observational drawings studying gestures are specifically life drawings.
Illustration is everything remained - usually published pieces intended to convey a theme or a concept.