Riba

Riba (リバ) is a term used both in Gay & Lesbian culture as well as Yaoi (Boys' Love) and Yuri (Girls' Love) fiction that refers to characters being versatile (adaptable or reversible) in the type of dominant or submissive role they can adopt.

Used in yaoi pairing referring to one whom contains mixed traits (回) of a seme/tachi and uke/neko.

Origin of Riba (リバ)
The word riba is probably just the abbreviation of the katakanization of the word "reversible," ribaashiburu (リバーシブル) or something like that.