What is Mother?

Legal Definition
A mother is the female parent of a child. Mothers are women who inhabit or perform the role of bearing some relation to their children, who may or may not be their biological offspring. Thus, dependent on the context, women can be considered mothers by virtue of having given birth, by raising their child(ren), supplying their ovum for fertilisation, or some combination thereof. Such conditions provide a way of delineating the concept of motherhood, or the state of being a mother. Women who meet the third and first categories usually fall under the terms 'birth mother' or 'biological mother', regardless of whether the individual in question goes on to parent their child. Accordingly, a woman who meets only the second condition may be considered an adoptive mother, and those who meet only the third a surrogacy mother.

The above concepts defining the role of mother are neither exhaustive nor universal, as any definition of 'mother' may differ based on how social, cultural, and religious roles are defined. The parallel conditions and terms for males: those who are (typically biologically) fathers do not, by definition, take up the role of fatherhood. Mother and fatherhood are not limited to those who are or have parented. Women who are pregnant may be referred to as expectant mothers or mothers-to-be, though such applications tend to be less readily applied to (biological) fathers or adoptive parents.
-- Wikipedia
Legal Definition
Domestic relations. A woman who has borne a child.

2. It is generally the duty of a mother to support her child, when she is left a widow, until he becomes of age, or is able to maintain himself; 8 Watts, R. 366; and even after he becomes of age, if he be chargeable to the public, she may, perhaps, in all the states, be compelled, when she has sufficient means, to support him. But when the child has property sufficient for his support, she is not, even during his minority, obliged to maintain him. 1 Bro. C. C. 387; 2 Mass. R. 415; 4 Miss. R. 97.

3. When the father dies without leaving a testamentary guardian, at common law, the mother is entitled to be the guardian of the person and estate of the infant, until he arrives at fourteen years, when he is able to choose a guardian. Litt. sect. 123; 3 Co. 38; Co. Litt. 84 b; 2 Atk. 14; Com Dig. B, D, E; 7 Ves. 348. See 10 Mass. 135, 140; 15 Mass. 272; 4 Binn. 487; 4 Stew. & Part. 123; 2 Mass. 415; Harper, R. 9; 1 Root, R. 487.

4. In Pennsylvania, the orphans' court will, in such case, appoint a guardian until the infant shall attain his fourteenth year. During the joint lives of the parents, (q. v.) the father (q. v.) is alone responsible for the support of the children; and has the only control over them, except when in special cases the mother is allowed to have possession of them. 1 P. A. Browne's Rep. 143; 5 Binn. R. 520; 2 Serg. & Rawle 174. Vide 4 Binn. R. 492, 494.

5. The mother of a bastard child, as natural guardian, has a right to the custody and control of such child, and is bound to maintain it. 2 Mass. 109; 12 Mass. 387, 433; 2 John. 375; 15 John. 208; 6 S. & R. 255; 1 Ashmead, 55.
-- Bouviers Law Dictionary
Legal Definition
A woman who has borne a child; a female parent; correlative to "son" or "daughter." The term may also include a woman who is pregnant. See Howard v. People, 185 III. 552, 57 N. E. 441; Latshaw v. State, 156 Ind. 194, 59 N. E. 471. See Mother-in-law.
-- Black's Law Dictionary