In graph theory, node A dominates node B if every path from the root node to B passes through A.
This concept is important for garbage collection because it means that B is only reachable through A. So if the garbage collector found A to be unreachable and eligible for reclaiming, than B would also be unreachable and eligible for reclaiming. So objects that A dominates contribute to the retained size of A: that is, the total amount of memory that could be freed if A itself were freed.
- Dominator on Wikipedia