iddah

In Islam, iddah or iddat (Arabic: العدة; period of waiting) is a period after a divorce, during which a woman may not marry another man. The time period is calculated on the number of menses a woman has, usually three. Historically, iddah was required to ensure that a male could know of any offspring produced after the cessation of a nikah, and be certain as to the parentage.
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k.r(Islam) 00-day waiting period before a widow or divorced woman may remarry

Waiting period of a widow.

Useful information: If a woman is not pregnant and her husband dies she should observe iddah (the waiting period) for four months and ten days (i.e. she should not marry during that period), even though she may be menopause, or her husband may have contracted fixed time marriage with her, or he may not have had sexual intercourse with her. In case, however, she is pregnant she should observe the waiting period till the birth of the child. But if the child is born before the expiry of four months and ten days from the death of her husband she should wait till the expiry of that period. This period is called the waiting period of death (Iddatul wafat).