Step Boyfriend 

A man who dates a woman that has children by another man. The man is the boyfriend to the mother of the children and thus a Step Boyfriend to the children.

The children left their Dad's house to go visit their Mom and Step Boyfriend for the weekend.

Step Boyfriend by Glitterfly78 December 3, 2009

Step Boyfriend 

A guy that isn't your boyfriend but talks to you like a boyfriend would.

"Me and Tristan have been talking, but we aren't official"

"So he's like a step boyfriend?"

Step Boyfriend by Astral21 April 22, 2021