concubine (n.)

c. 1300, "a paramour, a woman who cohabits with a man without being married to him;" also, in reference to Hebrew, Greek, Roman and other civilizations where the position was recognized by law, "a wife of inferior condition, a secondary wife," from Latin concubina (fem.), concubinus (masc.) "one who lives unmarried with a married man or woman." Usually the concubine was of a lower social order, but the institution, though below matrimonium, was less reproachful than adulterium or stuprum. The word itself is from concumbere "to lie with, to lie together, to cohabit," from assimilated form of com "with, together" (see con-) + cubare "to lie down" (see cubicle).

Such concubines were allowed by the Greek and Roman laws, and for many centuries they were more or less tolerated by the church, for both priests and laymen. The concubine of a priest was sometimes called a priestess. [Century Dictionary]

In Middle English, as in Latin, sometimes used of a man who cohabits with a woman without marriage. Related: Concubinary; concubinal.