IPA: /ˈstɛp.sən/
KK: /ˈstɛpˌsʌn/
A boy or man who is the son of one's spouse from a previous relationship.
My stepson loves to play soccer on the weekends.
Stepson is formed from "step" (from Old English *stæpe*, meaning a step or degree) and "son" (from Old English *sunu*, meaning a male child). The word refers to a male child of one's spouse from a previous relationship, indicating a familial relationship that is not biological but rather through a step or degree of marriage.
Think of a 'step' as a degree of relationship, and 'son' as a male child — a stepson is a male child related through a step in family, not by blood.