Unlawful sexual intercourse between two people not married to each other. In the scriptures it is also occasionally used as a symbol for apostasy.

Do not force me, for no such thing ought to be done, 2 Sam. 13: 12. Abstain from fornication, Acts 15: 20. The body is not for fornication, but for the Lord, 1 Cor. 6: 13-18. To avoid fornication, let every man have his own wife, 1 Cor. 7: 2-3. This is the will of God, that ye should abstain from fornication, 1 Thes. 4: 3. Jacob warned the people of Nephi against fornication, Jacob 3: 12. Ye are ripening, because of your murders and fornication, for destruction, Hel. 8: 26. Fornicators must repent in order to join the Church, D&C 42: 74-78.