Functionalism
What makes something a mental state is the role it plays, not what it is made from. All mental states are functional roles which can be multiply realised.
Functional role
By saying a mental state is a functional role we mean that it is specified in terms of
(1) what it is caused by (whether that is external causes or other mental states)
(2) what it causes (whether that is external behaviours or other mental states)
Multiple realised
many different kinds of state/event/thing can perform that functional/causal role.