step‧moth‧er /ˈstepmʌðə $ -ər/ noun [countable]
a woman who is married to your father but who is not your mother

[TahlilGaran] Dictionary of Contemporary English ▲

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

[TahlilGaran] English Thesaurus ▲