D&C 132:34-43; Mosiah 11:2, 4
“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.” (Bruce R. McConkie, Mormon Doctrine, p. 154)
“By definition, a concubine would be either a woman kept for lewd purposes or a lawful wife of a lower social standing than her husband’s other wife or wives (see also Mosiah 11:2). Hagar, plural wife of Abraham, would be an example of the latter, inasmuch as Abraham did only that which he was commanded (D&C 132:37). The offense to which Jacob made reference was the Nephites’ consorting either with paramours or with wives improperly taken.” (Robert Millet and Joseph McConkie, Doctrinal Commentary on the Book of Mormon, 2:20)