A Japanese abbreviation for "YAmanashi, Ochinashi, Iminashi" which translates to "without climax, without resolution, without meaning," and by the Japanese definition refers to doujinshi that support a non-canon,
homosexual (male/male) couple and are very explicit.
In English terms, however, the word is much more broad and can refer to anything with any
homosexual(only male/male) or boy's love themes in anime and manga, sometimes even expanding to non-anime/manga works, but often in connection with
fanfiction.