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.
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. The term is not generally used (but can be in individual cases) to refer to the relationship with an adult child who never lived in the home with the parent's new partner.
A stepfamily can be "simple," in which only one member of the couple has a prior child or children, or it can be "complex" or "blended." In a blended stepfamily, both members of the couple may have had pre-existing children, or the couple may have additional children together.
If both members of the couple have prior children, the children are stepbrothers and stepsisters to one another. Any subsequent child born to the couple becomes the half-sister or half-brother of the prior children from previous relations.