Boyd’s Bible Dictionary:
(ad-to and alter, other). Under Hebrew law the crime of unchastity, wherein a man, married or single, had illicit intercourse with a married or betrothed woman, not his wife. Punished with fire (Gen. 38:24); by stoning (Deut. 22:22-24). In a spiritual sense, apostasy.
Concise Bible Dictionary:
This was forbidden in the ten commandments; but neither there nor anywhere else is the sin defined. It seems clear, that as far as the man was concerned, if he had intercourse with a woman unless it was with a married woman, he would not be charged with adultery, though he himself might be married; indeed how could he be when he was allowed more wives than one, as well as concubines and slaves? If he committed adultery with a married woman or with one betrothed, both were to be put to death (Deut. 22:22-24). With the woman it was stricter, she must have no intercourse with any man but her husband. If a man was jealous of his wife there was the ordeal of the bitter waters provided to test her innocence (Num. 5:11-31). But we do not read that any man or woman was stoned for adultery, nor that any woman drank the bitter waters. We know from the New Testament that Moses had, because of the hardness of their hearts, allowed a certain looseness, and a man could divorce his wife for any cause, which was easier than bringing a suspected wife to trial. It may be that the men themselves had not good consciences, like those who brought the adulterous woman to the Lord in John 8:3. We have a dreadful picture of guiltiness in Judges 19, and Jeremiah charges Israel with being “as fed horses in the morning, every one neighed after his neighbor’s wife,” which loudly called for judgment (Jer. 5:8; 13:27). The Lord declared that a man morally committed adultery (or fornication) in his heart if he lusted after a woman. Adultery had also a typical meaning. Israel had been espoused to Jehovah, but instead of being a faithful wife she had sought other lovers. “With their idols have they committed adultery” (Ezek. 23:37). So the false church, who has Jezebel in her midst, the Lord will cast her “and them that commit adultery with her into great tribulation, except they repent of their deeds” (Rev. 2:22).
Strong’s Dictionary of Hebrew Words: