The traditional and strictest definition of a “stepfamily” is a married couple where one or both members of the couple have pre-existing children who live with them. More recently, the definition is often expanded to include all cohabiting couples, whether married or not. Some people also apply the term to non-custodial relationships, where “stepparent” can refer to the partner of a parent with whom the child does not live.
A stepfamily, also known as a blended family or reconstituted family, is a family in which one or both members of the couple have children from a previous relationship. The member of the couple to whom the child is not biologically related is the stepparent, specifically the stepmother or stepfather.