In an event-driven state machine…

Events -> Reactions ->Other Events

And as functors are to functions, Reactors are to reactions, namely, a class that behaves as a reaction.

State Reactions accept events as an input, and output events. They are scoped to the lifetime of the state that declares them.

This is in contrast to states, which also accept events as input, but then output transitions and parameter changes (important for State Machine determinism).