foster mother; adoptive mother
Refers to a woman who raises a child not biologically her own, either through fostering or adoption. The term is neutral and commonly used in legal and everyday contexts.
She is my foster mother.
養母との関係はとても良好だ。
My relationship with my adoptive mother is very good.
養母 refers only to the adoptive mother, whereas 養父母 covers both adoptive parents together.
Compound of 養 (よう, 'foster, nurture') and 母 (ぼ, 'mother'). The kanji 養 carries the meaning of raising or supporting, and 母 is the standard character for mother.