Buttfuckin' between two anime bishonen. Basically. That's about it.

Anime homosexuality

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.

A Japanese acronym for YAma nashi, Ochi nashi, Imi nashi, which means, roughly, "No peak, no point, no meaning". It is a term used to describe purely sex-focused manga or anime. It is often used incorrectly as a blanket term to cover yaoi, shounen ai, tanbi, shota, and BL, as well as being (again, incorrectly) used as a synonym for gay.

the word for homosexal relationships, usually only used to define relationships between anime/manga characters

Fictional homosexual pairings, usually somehow related to Japanese culture. A.k.a. drawings and stories of male characters giving each other head and fucking each other in the ass. Even though the content is adult, explicit, gay male sex, pretty much the only people who create or appreciate yaoi are straight female children. Don't ask me why.

When you are trying to type in “yaoi” but then auto correct tried to do its best* at “helping”

*there is no such thing as good autocorrect suggestions