a prefix used before father, mother, brother, sister, son, daughter, child, ·etc., to indicate that the person thus spoken of is not a blood relative, but is a relative by the marriage of a parent; as, a stepmother to x is the wife of the father of x, married by him after the death of the mother of x. ·see stepchild, stepdaughter, stepson, ·etc.
STEP-
Meaning of STEP- in English
Webster English vocab. Английский словарь Webster. 2012