IPA: /ˈstɛpˌtʃaɪld/
KK: /ˈstɛpˌtʃaɪld/
A child of a person's spouse from a previous relationship, who is not the biological child of that person.
She treats her stepchild as if they were her own.
Stepchild → It is formed from "step" (from Old English *stæpe*, meaning a step or degree) and "child" (from Old English *cild*, meaning a young human). The term "stepchild" refers to a child of one's spouse from a previous relationship, indicating a familial relationship that is a step away from direct descent.
Think of a 'step' as a degree of relationship, and 'child' as a young person. A stepchild is a child who is a step away from being your own.
No commonly confused words.