A step parent is someone who assumes the role of parent, is married to one of the biological parents, and is not a biological parent. Sometimes this person is more of a parent to the child than the biological parent, sometimes this person is not.
My biological dad's fourth wife is my step mother. The guy I call dad is my biological mother's second husband.
A parent that marrys someone in whom he or she falls in love with, who after being tortured with no sex, has to deal with the offspring of another while they tear the neighbohood apart with the protection of the biological parent.