mother-in-law; stepmother; foster mother
Refers to a mother by marriage or legal relationship, not by blood. Context determines whether it means mother-in-law, stepmother, or foster mother.
I live with my mother-in-law.
She is my stepmother, my father's second wife.
Compound of 義理 (giri, 'duty, obligation, relationship by marriage') + の (no, possessive particle) + 母 (haha, 'mother'). Literally 'mother of obligation/relationship'.