Stub

From BonsaiWIKI

A stub is a short length of branch or trunk that is left after pruning. Certain bonsai tools, such as concave cutters are designed to eliminate the stub..

In certain cases, a stub is left deliberately after pruning in anticipation of dieback, otherwise stubs are generally to be avoided as the practice of bonsai is to minimize the appearance of pruning wounds and anything that would diminish a tree's natural appearance.