A stepmother is the wife of someone's father, who is not their real mother.
Stepmothers are often thought of as mean in old stories, but some stepmothers are very nice.
Examples:
My stepmother makes good food.
I live with my stepmother and my dad.
The stepmother takes care of the children.
The stepmother in the story was very mean.
My stepmother has a magic wand.
The stepmother loves her stepchildren very much.