A stepfather is the husband of someone's mother by a later marriage. If a mother gets a divorce from the child's father or becomes a widow, she may marry another man who becomes her child's stepfather.
A stepfather and his stepchildren are not genetically related. It is also possible that the child's stepfather has children from a previous marriage. In this case, the stepfather's children become the stepbrother or stepsister of the mother's children.
The traditional definition of a stepfather has applied to a married man who has children from his wife's previous relationship living with him. Recently, however, this definition has expanded and can now be used to refer to a cohabiting couple, even if the mother does not marry her new partner. Some families even use the term for the new father of a non-custodial mother. It is rarely used, however, to describe a mother's husband who has never lived in a home with the mother's children.
It is possible for a stepfather to legally adopt his stepchildren. In this case, the man becomes the children's legal guardian and often chooses to stop using the terms stepson or stepdaughter to refer to his children. The children may also drop the step- and refer to the man as their father if they choose to do so.