Zina

Zināʾ (زِنَاء) or zina (زِنًى or زِنًا) is an Islamic law concerning unlawful sexual relations between male and female who are not married to one another through a "nikah". It includes extramarital sex and premarital sex. It also includes adultery (consensual sexual relations outside marriage). Zina covers fornication (consensual sexual intercourse between two unmarried persons), and homosexuality (consensual sexual relations between same-sex partners).
In the four schools of Sunni fiqh (Islamic jurisprudence), and the two schools of Shi'a fiqh, the term zināʾ is a sin of sexual intercourse that is not allowed by Sharia (Islamic law) and classed as a hudud crime (class of Islamic punishments that are fixed for certain crimes that are considered to be "claims of God"). To prove an act of zina, a qadi (religious judge) in a sharia court relies on an unmarried woman's pregnancy, the confession in the name of Allah, or four witnesses to the actual act of penetration. The last two types of prosecutions are uncommon. Most cases of zina in the history of Islam have been pregnant unmarried women. In some schools of Islamic law, a pregnant woman accused of zina who denies sex was consensual must prove she was raped with four eyewitnesses testifying before the court. This has led to many cases where rape victims have been punished for zina. Pressing charges of zina without required eyewitnesses is considered slander (Qadhf, القذف) in Islam, itself a hudud crime.
The above sense of "zina" is not to be confused with the woman's name "Zina" or "Zeina" (زينة). The name has a different linguistic root (Greek "xen"-). It also has a different meaning ("guest, stranger"), is pronounced differently (either Zīnah or Zaynah), and is usually spelled differently.