Stepchild (noun)

1. A child of one's spouse from a previous marriage or relationship.
2. A child who is not the biological or adopted child of one's spouse, but is considered to be part of the family.

Origin:

From step + child.

Examples:

1. She was always treated as a stepchild by her stepmother.
2. He felt like a stepchild in his own home, never really feeling like he belonged.
3. She was very close to her stepchild, who she had raised since he was a baby.
4. The stepchild was treated differently from the biological children.
5. The stepchild was not legally adopted, but was considered a member of the family.