Reactor
Short answer
An event handler in AutoCAD that fires when something happens — an object is modified, a command runs, a drawing is saved.
A reactor is AutoCAD's event-listener mechanism. Reactors come in flavours: database reactors (an entity was modified), command reactors (a command started/ended), document reactors (a drawing was opened/saved), and object reactors (this specific entity changed). LISP uses vlr-* functions to register reactors; .NET uses event handlers on Database, Document, and DocumentCollection. Reactors are how plugins implement 'this stays in sync automatically' behaviour.