Iddat is a period during which a Muslim woman is prohibited from marrying on dissolution of marriage:

Iddat is a period of waiting that a Muslim woman must observe after dissolution of marriage, whether it is by death or divorce, before she can remarry. During this period, which is usually three menstrual cycles or three lunar months, the woman is considered to be still married and is prohibited from marrying anyone else. The purpose of iddat is to ensure that the woman is not pregnant at the time of remarriage, and to give time for any dispute regarding the dissolution of the marriage to be resolved.