The biblical definition and usage of the word concubine is not the same as current dictionary usage. A current dictionary defines a concubine as "a woman living in a socially recognized state of concubinage" and then defines concubinage as "cohabitation of persons not legally married." (Webster's, p. 232.) In biblical times, a concubine was a legally married wife, usually of a second class status either because of social order, economic position, race, or nationality (a foreigner). Concubines had legal status and rights as wives, although not always of the same order or magnitude as the rights of "first wives."
It should be remembered that concubines in Old Testament times were part of polygynous marriages, where the husband could legally have more than one wife at the same time.
COMPANION TO YOUR STUDY OF THE OLD TESTAMENT
by Daniel H. Ludlow
See CELESTIAL MARRIAGE, PLURAL MARRIAGE. In modern times a concubine is a woman who cohabits with a man without being his wife. But "from the beginning of creation," all down through the history of God's dealings with his people, including those with the house of Israel, concubines were legal wives married to their husbands in the new and everlasting covenant of marriage. (D. & C. 132:1, 37-39, 65.)
Anciently they were considered to be secondary wives, that is, wives who did not have the same standing in the caste system then prevailing as did those wives who were not called concubines. There were no concubines connected with the practice of plural marriage in this dispensation, because the caste system which caused some wives to be so designated did not exist.
Mormon Doctrine
Bruce R. McConkie