A sexual partner outside of your primary relationship in which the partner "supplements" what your main relationship is lacking in. The term is used as a less crass version of "fuck buddies" or "friends with benefits."
"My husbandwon't go down on me, but my supplementation partner sure does."
Boys who enjoy yaoi (a genre in Japan that contains sexual and/or romantic relations between two men); literally translates to "rotten boy"; corresponding female : fujoshi