A Japanese abbreviation for "YAmanashi, Ochinashi, Iminashi" which translates to "without climax, without resolution, without meaning," and by the Japanese definition refers to unofficial comic books (doujinshi) that support a non-official, 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.
It is important to note that although the word 'homosexual' has been mentioned quite a bit in this definition, much of Japanese yaoi manga, as with most BL (boy's love) manga, cannot truly be considered homosexual content. This is because the men/boys in the comic, although often clearly represented in a relationship together (usually physical in nature), are in fact not homosexual, but show a deep romantic connection only to the one man they are portrayed as in love with. (Although this is not ALWAYS the case.)
"I hope that new Naruto/Sasuke doujin comes in soon, I'm afraid I'm suffering from a bit of yaoi withdrawl."
"Did you see that new Frodo/Sam yaoi fic? It was pure smut from start to finish."