stepmother
step‧moth‧er /ˈstepmʌðə $ -ər/ BrE AmE noun [countable]
a woman who is married to your father but who is not your mother
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THESAURUS
▪ mother a female parent: My mother and father are both teachers.
▪ mum British English informal, mom American English informal used when talking to your father, or about someone's father: My mum and dad won’t mind if you want to stay the night. | Mom, where’s my sweater?
▪ mummy British English, mommy American English a name for mother, which is used especially by young children or when you are talking to young children: Where’s Mummy, Abbie?
▪ ma American English old-fashioned used when talking to your mother, or about someone's mother: Ma stirred the soup on the stove.
▪ mama old-fashioned used when talking to your mother, or about someone's mother: Mama seldom disagreed with Papa.
▪ stepmother (also stepmum British English informal, stepmom American English informal) a woman who is married to your father, who is not your mother but often acts as your parent: the wicked stepmother in fairy stories