The word adultery is etymologically related to the word adulterate, which means “to render something poorer in quality by adding another substance.” Adultery is the adulteration of marriage by the addition of a third person. Adultery is voluntary sexual activity between a married person and someone other than his or her spouse.

The Bible begins its teaching on marriage with the pattern of Adam and Eve: one man and one woman, husband and wife, united by God (Genesis 2:24, Mark 10:7–9). Adultery is forbidden by the seventh commandment: “You shall not commit adultery” (Exodus 20:14). The fact that the prohibition is simply stated with no explanation indicates that the meaning of adultery was well understood at the time Moses gave the law. Scripture is consistent in the prohibition against adultery.