Adultery (informally known as cheating in non-religious situations) is a word used in religious texts like Exodus 20:14. Adultery applies to a married person having sex with someone other than the person who they are married to.
In countries where religion is important, adultery often comes with a heavy punishment. Some Muslim countries punish it with death by stoning. In most countries adultery is no longer a crime, but most people still see it as a bad thing.(see Gospel of John 8) If a person who is married takes part in adultery, that person's husband or wife would usually have the right to be able to go to court to divorce them.