Marriage (nikah) is a solemn and sacred social contract between bride and groom. This contract is a strong covenant (mithaqun Ghalithun) as expressed in Quran 4:21). The marriage contract in Islam is not a sacrament. It is revocable.
Both parties mutually agree and enter into this contract. Both bride and groom have the liberty to define various terms and conditions of their liking and make them a part of this contract.