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 (Exd 21:7; Deu 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 (Exd 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 (Mat 19:5-9; 1Cr 7:2).