Other forms: reginas
You might know it best as a female given name, but regina is a Latin term meaning "queen." In English, it's used in specific legal, historical, and biological contexts.
In constitutional monarchies like the U.K., the term regina, often capitalized or abbreviated as R, refers to the reigning monarch (if a queen is on the throne) in legal proceedings, as in a court case titled R v. Smith. (If the monarch is a king, the R stands for Rex, "king.") In older biological texts, the queen bee in a colony may be called the regina. And the term appears in scientific nomenclature, as in the North American snake genus Regina, which includes the queen snake, Regina septemvittata.