Zina (Arabic: زنا / ALA-LC: zinā) is generally defined by Islamic Law as unlawful sexual intercourse, i.e. intercourse between a man and a woman who are not married to one another. This encompasses extramarital sex and premarital sex.
Zina falls under the Islamic sexual jurisprudence of Fiqh, which is an expansion of the Sharia code of conduct given in the Qur'an.
Across all four schools of Sunni practice, and the two schools of Shi'a practice, the term zina signifies voluntary sexual intercourse between a man and a woman not married to one another, regardless of whether one or both of them are married to other persons or not. It does not - in contrast with the usage prevalent in most Western languages - differentiate between the concepts of "adultery" (i.e., sexual intercourse of a married man with a woman other than his wife, or of a married woman with a man other than her husband) and "fornication" (i.e., sexual intercourse between two unmarried persons).
Islamic law prescribes punishments for both Muslim and non-Muslim men and women for the act of Zina as interpreted from the Qur'an and the Hadith. In principle it is an extremely difficult offence to prove, requiring four respectable witnesses to the actual act of penetration.