The term 2.5-D was coined by anime fans in Japan in the 1980s to refer to animes voice actors, but in the 2000s it began to refer to some cultural practices exercised in a space between 2-D fiction and 3-D reality. Thus, the 2.5-D culture is cultural practices which produce the fictional space of contemporary popular cultural products (such as manga, anime, and videogames) along with the fans interplay between the real and fictional spaces.
Its examples are: 2.5-D theaters (theatrical adaptation of anime, manga, and videogames), cosplay, contents tourism (pop-culture-induced tourism), character/voice actor concerts (ex. concerts of Love Live! and Ensemble Stars, etc.), enjoei (a cheer-a-long style of movie screening), and V-tubers (virtual YouTubers). What matters in these cultural products are active interactions between the reality of characters of anime/manga/videogames and the virtuality of the human bodies of practitioners (actors and fans). As in Henry Jenkinss convergence culture,” 2.5-D culture is generated across multiple transmedia platforms, cooperation of multiple media industries, and fans migratory behaviors. Fans actively migrate among the fictional, cyber, and physical worlds.kottke.orgThe Second and a Half Dimension