Nikah, or nikkah, (Arabic: النكاح ‎), is the contract between a bride and bridegroom and part of an Islamic marriage, a strong covenant (mithaqun Ghalithun) as expressed in Qur'an 4:21.

Nikah is based on few conditions. When these conditions are fulfilled, then a man and a woman are proclaimed as husband and wife and can live together and carry on their marital duties. These are:

(1) Giving free consent to the marriage personally or through a close relative like father (called Wali). A man and woman say in clear voice three times that they accept (name of the person and his/her father name is called) as their husband/wife. (2) The amount of Haq-e-Mahr (marriage money) is agreed upon, announced, and paid to the bride at the spot or paid in the future. (3) Two adult free men witness the ceremoney. (4) The nikah is publicised/advertised widely.

In some engagement ceremonies, all the above four conditions are fulfilled. Therefore, such engagements are full-fledged nikahs.

The nikah contract requires the consent of both parties very strictly if they are adults. There is a tradition in some Muslim countries, outside of the religion, to pre-arrange a marriage for young children. However, the marriage still requires consent of the concerened man and woman when they reach adulthood when the wedding actually goes ahead. Islam does allow divorce (talaq) so this contract is revocable. Marriage is seen as a necessity in Islam and is seen as helpful in avoiding zina (extramarital sex) or cruelty. A marriage which is carried out unofficially without informing the public is called nikah urfi.