In the English use of the word, a child neither born nor begotten in lawful wedlock; an illegitimate child. There is no Hebrew word of like meaning. The mamzer, rendered "bastard" in the A. V., is something worse than an illegitimate child. He is the offspring of a father and mother between whom there could be in law no binding betrothal: issuing either from adultery between a married woman and a man other than her husband, or from incest within the forbidden degrees of kinship or affinity defined in Lev. xviii. and xx. The child of a marriage simply forbidden, as that between a cohen and a divorced woman, is legitimate but "profane"; that is, a son can not officiate as a priest, a daughter is not eligible to marry a priest. But a mamzer, according to Deut. xxiii. 3, must not "enter the congregation of the Lord," that is, marry an Israelite woman, "nor shall his tenth generation enter," etc., which includes also the female mamzer (Ḳid. iii. 12; Mak. iii. 1). The older Halakah, however, was more rigorous, Akiba declaring any child of a forbidden connection a mamzer (Yeb. iv. 12, 13; Yer. ib. 6b; Bab. ib. 44a, 49a).
Whether the child of a daughter of Israel and of a Gentile or bondman is a mamzer or not, was hotly disputed both among the early sages, down to Rabbi Judah the Patriarch, and among the later teachers in Palestine and in Babylonia (Yeb. 23a, 45a). But the rule finally adopted is that such a child is not a mamzer, even when the mother is a married woman. This is the decision in the modern code (Shulḥan 'Aruk, Eben ha-'Ezer, 4. 19), though it is admitted that the child is unfit for the priesthood. Maimonides decides to the same effect (Issure Biah, xv. 3). The law laid down in Deuteronomy against the mamzer and against his distant offspring seemed so harsh that every opportunity was taken to confine it to the narrowest limits.
Where incest or adultery takes place among Gentiles, and the offspring embraces Judaism, the flaw in his descent is ignored. He is not deemed a mamzer (Shulḥan 'Aruk, Eben ha-'Ezer, 4, 21). The child of an Israelite by an unconverted Gentile mother is a Gentile, and when converted becomes an Israelite to all purposes, without regard to his father.
As shown under Agnates, the illegitimate child of a Jew (unless born of a Gentile woman or a bond-woman), even a mamzer, inherits from his natural father and other kindred (for example, his father's legitimate sons), just as if he were legitimate; the words of Scripture, "if he have no son" (Num. xxvii. 8), being taken literally "a son from any source," except the son of a Gentile or bondwoman, who follows the status of his mother (Yeb. ii. 5); and the child being bound by all duties flowing from his or her natural kinship.
This construction of the law runs counter to ancient popular sentiment, which crops out in the historic books. The legitimate sons of Gilead drove Jephthah from his home because he was the "són of another woman" (Judges xi. 2). Where a child is born in wedlock, the presumption in favor of its being the offspring of the husband is very strong, as in other systems of law. The Roman law says: "pater est quem justæ nuptiæ demonstrant." But the Jewish law, unlike the English common law, does not uphold this presumption when the child is born so soon after the nuptials ("nissu'im") that it must have been begotten before them. Even when the date of birth points to conception after the betrothal ("erusin")—which in olden times preceded the wedding by several months—the presumption of the betrothed man being the father is comparatively weak, as a connection between him and the bride while she is "at her father's house," though not a deadly sin on the part of either, is an act of lewdness (Shulḥan 'Aruk, Eben ha-'Ezer, 4, 27; see Ket. 36a).
On the general principle that a person's confession of his or her own turpitude is not admissible as legal testimony, the wife and mother can not, by her assertion, stamp her offspring as an adulterine Bastard. For the rules of presumption and evidence in cases of doubt, see Shulḥan 'Aruk, Eben ha-'Ezer, 4, 14-16.