Definition of fornication:
(n) :
sexual intercourse, especially on the part of an unmarried person; the act of such illicit sexual intercourse between a man and a woman which does not by law amount to adultery
"1604 I am the sister of one Claudio, Condemn'd upon the act of fornication To lose his head; condemn'd by Angelo -- Shakespeare, Measure for Measure, Act 5, Scene 1"