A term used to describe the first born child of a divorced man and/or woman who remarries. Typically when the new couple comes together they already have children from a previous relationship. Together the new couple will have at least one biological child of their own in order to "cement" the relationship together in an attempt to make them feel more like a legitimate family.
"Great... Mom just married some old guy who already has 5 kids of his own. Well any attention that should have been paid to us kids is now going to go to their cement baby instead. Back burner and emotional scaring... here we come."