Adultery is the crime of having sexual intercourse with (expanded by Jesus to simply lusting after) someone else's spouse or someone who is not your spouse. The seventh commandment from God is to refrain from this behavior.

Biblically, adultery is not just spoken of as a sexual betrayal between married people. Adultery is the term God uses to reference Israel’s idolatry, pointing back to the first and second commandments:

Many people seek reason for God's law and are not satisfied with simply following the rules, so let’s talk about the consequences for breaking this commandment. An obvious consequence of our day is that you increase your risk of contracting a sexually transmitted disease. Also, before the 1980s it was nearly impossible to identify paternity outside of he-said she-said.[1] If a woman had multiple partners, when she became pregnant there was no way of knowing which partner was the father (unless one of the potential fathers was from a different race and the baby was born with features from that race). Also, jealousy would become a problem. Both men and women are prone to jealousy over their spouses being wooed by someone else. This jealousy often leads to violence.

An interesting thing is whether or not polygamy is considered adultery... Arguments have been formed both for and against polygamy.[2][3][4] Obviously, those who are against polygamy consider this to be adultery, while those who are for polygamy do not consider it to be adultery. Lamech, from Cain's line, was the first man mentioned to have two wives, which doesn't give the notion much credibility, but several of God's chosen people have multiple wives as well: Abraham and Jacob for example. In Abraham's case, God protected Hagar and Ishmael but the blessing of His covenant was passed on to Isaac, the son of Abraham's first wife. While Joseph, Jacob's son from his second wife, experiences both hardship and prosperity as the favored son, it is through Judah, Jacob's son from his first wife, that King David and Jesus are descended. Those who are pro-polygamy cite Moses' law in Deuteronomy 21:15 concerning a man with two wives as proof that polygamy was not condemned by God. Of course in the New Testament, we are told a blameless man should be a man with one wife (1 Timothy 3:2, 1 Timothy 3:12, 1 Timothy 5:9, and Titus 1:6).

So, is polygamy ok, is it adultery? I would say, based on the verses in the Bible that discuss marriage, that polygamy is/was not God's preferred marriage institution, but also wasn't expressly forbidden. If God wanted polygamous relationships from the beginning, He would have created multiple wives for Adam. Just as early humans were allowed to marry their siblings but later forbidden, this was something God allowed to occur. In each of these scenarios, we see problems that occur not only between wives (Hagar vs. Sarah, Rachel vs. Leah) but also in their descendants. I would wager that polygamy, when legally possible, is not adultery—but I wouldn't suggest running out to take extra wives (or husbands) either.

The most famous example of adultery in the Bible is that of David seducing Bathsheba. Not only does he commit adultery, but when she becomes pregnant from their infidelity, David tries to trick her husband into sleeping with her. Clearly, he planned to lie about the paternity of the baby (another sin). When this fails, he has Bathsheba's husband killed. The problem with most sins is that they lead us to commit other sins!