The Catechism in Questions & Answers

342. What is the meaning of the Sixth Commandment?

Marriage is the lifelong union between a man and a woman as desired by God. It is based on an act of free will which is expressed in a public vow of fidelity.
Any married individual who has sexual intercourse with anyone other than his / her spouse commits adultery. Likewise, anyone who has sexual intercourse with a married person commits adultery.