Dictionary.net

A child or spouse who is not mentioned in a will and whom the court believes was accidentally overlooked by the person who made the will. For example, a child born or adopted after the will is made may be deemed a pretermitted heir. If the court determines that an heir was accidentally omitted, that heir is entitled to receive the same share of the estate as she would have if the deceased had died without a will. A pretermitted heir is sometimes called an "omitted heir."

the name applied to a person who was born after a dead person executed his will but before he died.