'Concubine' definitions:

Definition of 'concubine'

noun

A woman who cohabits with an important man [syn: concubine, courtesan, doxy, paramour]

From: GCIDE

Concubine \Con"cu*bine\, n. [F., fr. L. concubina; con- + cubare to lie down, concumbere to lie together, akin to E. cubit.]
1. A woman who cohabits with a man without being his wife; a paramour. [1913 Webster]
Note: Concubine has been sometimes, but rarely, used of a male paramour as well as of a female. --Trench. [1913 Webster]
2. A wife of inferior condition; a lawful wife, but not united to the man by the usual ceremonies, and of inferior condition. Such were Hagar and Keturah, the concubines of Abraham; and such concubines were allowed by the Roman laws. Their children were not heirs of their father. [1913 Webster]

Concubine in the Bible denotes a female conjugally united to a man, but in a relation inferior to that of a wife. Among the early Jews, from various causes, the difference between a wife and a concubine was less marked than it would be amongst us. The concubine was a wife of secondary rank. There are various laws recorded providing for their protection (Ex. 21:7; Deut. 21:10-14), and setting limits to the relation they sustained to the household to which they belonged (Gen. 21:14; 25:6). They had no authority in the family, nor could they share in the household government.
The immediate cause of concubinage might be gathered from the conjugal histories of Abraham and Jacob (Gen. 16;30). But in process of time the custom of concubinage degenerated, and laws were made to restrain and regulate it (Ex. 21:7-9).
Christianity has restored the sacred institution of marriage to its original character, and concubinage is ranked with the sins of fornication and adultery (Matt. 19:5-9; 1 Cor. 7:2).