an apparatus in which a chain reaction of fissile material, e.g. uranium or plutonium, takes place, is started and controlled, esp for the production of nuclear power or elementary particles; a nuclear reactor

a vat in which an industrial chemical reaction takes place

somebody who reacts, e.g. to a drug

a device, e.g. a coil of wire, in an electrical circuit that introduces reactance into the circuit, e.g. for dimming a light