Zina stands for both fornication and adultery, which are cardinal sins. Islamically, it refers to sexual intercourse between man and woman with no legitimate marital relationship.

Adultery

Definition and ruling

In Arabic, zina stands for both fornication and adultery. Linguistically speaking, it means having sexual intercourse outside marriage. In Islamic terminology, it refers to sexual intercourse between man and woman when there is no legitimate marital relationship, real or imagined, between them. 

Fornication and adultery are major sins.

 'Abdullah ibn Mas'ud reports: ‘I asked Allah’s messenger which sin is the gravest in Allah’s sight? He said: “To attribute an equal to Allah when He is the One who created you.” I said: “This is certainly grave. What comes next?” He said: “To kill your child so that he (or she) does not share your food.” I asked: “Which comes next?” He said: “To commit adultery with your neighbour’s wife.”’