IPA: /ˈstɛpˌmɒm/
KK: /ˈstɛpˌmɑm/
A woman who is married to someone's father but is not their biological mother.
My stepmom makes the best cookies for us.
Stepmom is formed from "step" (from Old English "stæpe", meaning a step or degree) and "mom" (a colloquial term for mother, derived from the word "mother" which comes from Old English "modor"). The term "stepmom" refers to a woman who is married to one's father after the divorce or death of one's biological mother, thus taking a step in the family hierarchy.
Think of 'step' as a degree or level in a family relationship, and 'mom' as a familiar term for mother. This helps you remember that a stepmom is a mother figure who comes into your life through a new marriage.
No commonly confused words.