Bisimulation. We have given the definition of bisimulation earlier, in Section 3.1. We discussed it in connection with graphs, but the reader may also know of a notion with the same name coming from modal logic. Actually, the theory of coalgebra studies a more general notion, that of bisimulation on a coalgebra for a given functor, defined first in Aczel and Mendler (1989). This more general notion specializes to several concepts which had been proposed in their own  fields.     In addition, it is (nearly) the dual concept of a congruence on an algebra; this explains our line in  the conceptual comparison chart. 