Boyd’s Bible Dictionary:

(lying with). In the Jewish economy, a secondary wife, betrothed according to custom (Gen. 21:14; 25:6; Ex. 21:7; Deut. 21:10-14). Concubinage repudiated in N. T. (Matt. 19:4-9; 1 Cor. 7:2-4).

Concise Bible Dictionary:

These were a class of inferior wives: they were at times personal servants given by wives to their husbands from their great desire for children, who then accounted the children of the servant as their own, as it was with Rachel and Leah. Such cases may have been comparatively rare, and would in no way account for the prevalence of men having concubines. Deuteronomy 21:11 gives the root of it: a man saw a beautiful woman and lusted after her. God seems to have simply allowed it: as the Lord said about their easy way of writing a bill of divorcement: Moses permitted it “because of the hardness of your hearts.” When God spoke of Israel having a king, one of the things forbidden to him was that of multiplying wives, lest his heart be turned away (Deut. 17:17). This alas, was the very fall of Solomon, who had 700 wives and 300 concubines, and they did turn away his heart (1 Kings 11:3). In the Canticles we read of 60 queens and 80 concubines and virgins without number; but there was one, a choice one, the only one of her mother, that excelled them all—the bride of the song (Song of Solomon 6:8-9). Esther 2:14 and Daniel 5:2 show that concubinage was a custom also among the heathen. Christianity disallows such evil, and recognizes the relationship as established of God, and hence the sanctity of the marriage tie in those whom God joins together.