stepparent

Definition from Nolo’s Plain-English Law Dictionary

The spouse of a parent, who becomes the stepparent of that parent's child upon marriage. Stepparents are not legal parents of their spouse's children unless they complete a stepparent adoption, which requires the consent of the other legal parent or the termination of that parent's rights. (See also: stepparent adoption)

Definition provided by Nolo’s Plain-English Law Dictionary.