from the Japanese verb 'ukeru' (receive) it's used in anime/manga and fanfiction for the "bottom" or "passive" in a yaoi/shounen-ai (male/male) relationship.
Generally the shorter, cuter, sweeter of the two... generally.
See seme for more info.

The Japanese term for the partner on the bottom. Used in conjunction with seme and yaoi.

partner on bottom, submissive one in relationship , usually younger and less experienced the the seme

The person on "bottom" or the submissive person during intercourse. Contrary to popular belief, this word is not always associated with sonen-ai or the likes thereof.

Often referring to yaoi manga, ukes are usually the bottom ones. Opposite of semes, ukes are usually cute, has less experience with sex, kind, those kinds of things.

during a guy x guy relationship the older male is the seme and the younger is the UKE meaning the younger is the female part in the relationship.

Alternative word to describe a Martial Arts partner, or attacker, who is the recipient of the technique to be practiced or attempted.

Also, can be the problem child from the class, i.e. doubting Thomas, to be used for an example to train them mentally. The Demonstratee.