lit. 'wrong or evil conduct with regard to sensual things'; 'unlawful sexual intercourse' refers to adultery, and to intercourse with minors or other persons under guardianship.
The abstaining from this unlawful act is one of the 5 moral rules (s. sikkhāpada) binding upon all Buddhists. Through any other sexual act one does not become guilty of the above transgression, which is considered a great crime. The monk, however, has to observe perfect chastity.
In many Suttas (e.g. A.X.176) we find the following explanation:
"He avoids unlawful sexual intercourse, abstains from it. He has no intercourse with girls who are still under the protection of father or mother, brother, sister or relatives, nor with married women, nor female convicts, nor, lastly, with betrothed girls."