The Greek word used in the New Testament for an adulterer is moichos (adultery is mocheia) and it refers to someone who is married but having sexual intercourse with someone other than his or her spouse. In the Old Testament, and in both the secular Greek and Roman worlds, the definition of adultery was different. In the Old Testament, “adultery” properly referred to a married or betrothed woman having sexual intercourse with someone other than her husband. The Greek, Roman, and Hebrew concept of adultery was substantially the same. “The infidelity of the husband did not constitute adultery.” 
James Hastings writes:
More specifically in the Israelite as well as Roman law, the term [adultery] was confined to illicit intercourse of a married or betrothed woman with any other man than her husband. Other unchaste relations were disapproved, but they were described by different words. 
William Smith adds some insight:
The parties to this crime [adultery] were a married woman and a man who was not her husband. The toleration of polygamy, indeed, renders it nearly impossible to make criminal a similar offence committed by a married man with a woman not his wife. 
Some theologians say that because God created Adam and Eve for each other, the Law implies that adultery was the same in the New Testament and the Old Testament. Although there is no question that God originally intended marriage to be a one-to-one relationship, there is also no question that polygamy started early, only seven generations after Adam (Gen. 4:19), and was provided for under Mosaic Law (Exod. 21:10; Lev. 18:18; Deut. 21:15). It is well known that the 12 tribes of Israel exist because Jacob had two wives and two concubines and fathered children from each of them. Many “great names” of the Old Testament had multiple wives and concubines, and did so without breaking the command of God. That would simply not be the case in Christianity. 
God does not explain why He provided for multiple wives in the Old Testament. It was obviously not His original intention for marriage, because He originally designed the “two” to “become one flesh.” The need for connection and “feeling special” is fundamental to any marriage relationship, and shows up powerfully in places like Song of Solomon. It is hard to imagine that the woman would easily be able to share the affection of the one her “heart loves” (Song of Sol. 3:1-5) with another woman. Furthermore, polygamy never did work out very well. There are no “happily married threesomes” in Scripture. The tension between Rachel and Leah is well known, and Peninnah is called Hannah’s “rival” in 1 Samuel 1:6. The Chinese pictograph for “fighting” is the symbol of two women under one roof, and harem intrigues and infighting are legendary.
It has been suggested that, in allowing polygamy, God was making a concession to the culture, but this explanation is not satisfactory. He did not make concessions about food, worship, cleanliness, criminal activity, the authority of leadership, or in other areas of sexual behavior such as adultery, marriage to family members, prostitution, homosexuality, etc., so the idea that He “gave in” when it came to the one area of multiple wives is surely not correct. The best explanation we have seen posited is that polygamy provided a way for a woman to become part of a household that had the strength and influence to provide for and protect her and her children.
The “bottom line” for people today is this: the New Testament specifically commands that each man and woman is to have his or her “own” spouse, and thus the New Testament definition of adultery is narrower than the Old Testament definition, including any married person having intercourse with someone other than his or her spouse. The world and its evil forces want to discount the evil in adultery, so it is called “an affair,” “cheating,” or “indiscretion,” as if it were no big deal. But it is a big deal, and God has always firmly commanded that people do not commit adultery. Verses forbidding it are in the Old Testament (Exod. 20:14, etc.), the Gospels (Mark 10:19, etc.), and in the Church Epistles (Rom. 13:9, etc.).
Adultery is a much more serious sin than sex before marriage because covenants, commitments, promises and expectations are involved. A marriage covenant is very serious in the eyes of God. Disregarding the covenants made before men and God and destroying the “one flesh” arrangement God created when “He created them male and female” is a very serious sin, one that often leads to the destruction of the family, and one that in the Old Testament was punishable by death (Lev. 20:10,11; Deut. 22:22). Married people need to take very seriously the words of Scripture concerning adultery: “Marriage should be honored by all, and the marriage bed kept pure, for God will judge the adulterer and all the sexually immoral” (Heb. 13:4). 
Adultery was clearly understood to be a sin, and so it was the perfect “tangible” example of the often less tangible sin of idolatry. Thus, in the Old Testament, words such as “adultery,” “sexual immorality,” “prostitute,” and “unfaithfulness” can have a literal meaning or a spiritual meaning. Adultery occurred when a woman was unfaithful to her husband and broke her marriage covenant. When God wanted to communicate the grievous nature of the sin of idolatry, he portrayed it as adultery, and Israel as an adulterous wife or a prostitute. God was to be Israel’s “one and only,” and if she worshipped other gods, she was being unfaithful. Furthermore, Israel had made covenants and commitments with God and promised that she would obey His commands (Exod. 24:7; Deut. 5:27; Josh. 24:14-25), and those covenants were broken when Israel worshipped other gods.
There is another reason that “adultery” was a good word to describe the worship of pagan gods: a lot of pagan worship involved sex. This shows up early in the Bible when Israel was at Sinai and made the golden calf, an Egyptian fertility god, and then “rose up to play,” a reference to the sexual acts involved with worship of that pagan god (Exod. 32:6, KJV). The connection of adultery and fornication with pagan worship makes for some interesting double entendres in the Bible. For example, in Jeremiah 13:27 God says He has seen Israel’s “detestable acts on the hills and in the fields,” and the context makes it clear that both spiritual adultery and physical adultery are indicated.
There are people (usually those involved in adultery and trying to justify it) who say that the adultery in the Bible is always spiritual adultery, but that assertion is proven false simply by reading a few scriptures. For example, Leviticus 20:10 says, “If a man commits adultery with another man’s wife…” which is a clear indication that the adultery was the physical relation between the man and woman. Other verses are similar. The introduction to Psalm 51:1 speaks of the adultery of David with Bathsheba. Jeremiah 29:23 mentions the people of Judah committing adultery with their neighbors’ wives, and of course there is the well-known record of the woman who was caught in the act of adultery and brought to Jesus. Adultery is a serious sin in the eyes of God, and Christians need be honest about that and make sure that their lives are pure in the sight of God.
 Charles R. Barnes, The People’s Bible Encyclopedia (The People’s Publication Society, Chicago, IL, 1921), p. 23.
 James Hastings, A Dictionary of the Bible (Hendrickson Publishers, Peabody, MA, 1988. Reprint of 1898 edition), Vol. 1, pp. 520,521.
 William Smith, Dictionary of the Bible (Baker Book House, Grand Rapids, MI, 1981. Reprinted from 1868 edition), Vol. 1, p. 36.
 A concubine was a woman taken into the family and generally thought of as a “lesser wife.” These women were usually slaves that were bought, war captives, or “gifts” given to a powerful man (many parents who wanted to be connected to a powerful family did so by giving a daughter as a gift—this probably happened often to Solomon, who had 700 concubines).
 Although both men and women commit adultery, it is interesting that surveys show that 80% of the married men who commit adultery say they still love their wives, while only 6% of the women say that. Men need to be serious about obeying God sexually and in loving their wives. The fact that only 6% of the women who commit adultery say they still love their husbands shows when a woman feels loved she will usually not disobey God and commit adultery (reported in Kosher Sex, p. 197).