What is the term for a piece of crust which has been in a collision along a convergent crustal boundary and become accreted to another crustal plate?

Answer: Terrane

Explanation:

In geology a terrane, or more precisely refered to by its complete name: a tectonostratigraphic terrane, is formed when a fragment from one tectonic plate breaks off and accretes itself to crust lying on another plate.

A terrane can be recognized because the crustal fragment keeps its own unique geologic history which different from that of the surrounding areas.