postumus [Roman law.] (1) A child of a testator, born after his death, who, if born in his lifetime, would have been under his potestas, and entitled to succeed him if he died intestate; (2) a child of a testator conceived before the date of the will, but born a suus heres after the date of the will, and before the testators death. This was called a postumus Vellaeanus, from lex Junia Vellaea, which provided that the testator might institute or exclude such a child.