When a Muslim man declares his wife to be like a Mahram (unmarriageable kin), it's a serious declaration known as Zihar (ظهار), effectively making her unlawful to him as if she were his mother or sister, but in Islamic law, it's a wrongful act requiring atonement (Kaffarah), often involving freeing a slave or fasting, to rectify the prohibited relationship, rather than a legitimate declaration of divorce or a new status.
When someone declared his wife just as the relation of maharmaat, that act is called.
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