Adultery in the Bible
The King James version of the Bible uses the word "adultery" 43 times. The Old Testament mentions adultery 20 times, while the New Testament uses it 23 times. And of course, there are many Bible verses that address adultery without using that word. So what does "adultery" mean in the Bible? We begin with the the Old Testament's first mention of adultery, in Exodus, then move to Deuteronomy.
The Ten Commandments
The Bible tells the story of the Ten Commandments twice, first in Exodus, then in Deuteronomy. The wording in the two Books is virtually identical:
Thou shalt not commit adultery. (Exodus 20:14)
Neither shalt thou commit adultery. (Deuteronomy 5:18)
This Commandment is usually referred to as the Seventh Commandment.
But there's another Commandment that speaks to adultery. It is the Tenth Commandment:
Thou shalt not covet thy neighbour's house, thou shalt not covet thy neighbour's wife, nor his manservant, nor his maidservant, nor his ox, nor his ass, nor any thing that is thy neighbour's. (Exodus 20:17)
Neither shalt thou desire thy neighbour's wife, neither shalt thou covet thy neighbour's house, his field, or his manservant, or his maidservant, his ox, or his ass, or any thing that is thy neighbour's. (Deuteronomy 5:21)
We see, then, that the act of having sex outside of marriage is forbidden by the Seventh Commandment, while the Tenth Commandment forbids lusting for another's spouse.
More of the Old Testament
The Old Testament emphasizes that adultery is committed when a married person has sex with someone not his or her spouse. For example, at Ezekiel 16:32 the following text appears: "But as a wife that committeth adultery, which taketh strangers instead of her husband!"
The Old Testament actually distinguishes between adultery and "whoredom:"
They sacrifice upon the tops of the mountains, and burn incense upon the hills, under oaks and poplars and elms, because the shadow thereof is good: therefore your daughters shall commit whoredom, and your spouses shall commit adultery. (Hosea 4:13)
I will not punish your daughters when they commit whoredom, nor your spouses when they commit adultery: for themselves are separated with whores, and they sacrifice with harlots: therefore the people that doth not understand shall fall.
From this it can be concluded that single people who have sex with other single people might be whores, but they have not committed adultery within the meaning of the Seventh Commandment.
The New Testament
Jesus expands on what adultery means in the New Testament when he states, "But I say unto you, That whosoever looketh on a woman to lust after her hath committed adultery with her already in his heart." (Matthew 5:28)
Perhaps Jesus is referencing the Tenth Commandment which forbids coveting a neighbor's wife.