/ree ak"teuhr/ , n.
1. a person or thing that reacts or undergoes reaction.
2. Elect. a device whose primary purpose is to introduce reactance into a circuit.
3. Immunol. , Vet. Med. a patient or animal that reacts positively towards a foreign material.
4. Also called atomic pile, chain reactor, chain-reacting pile, nuclear reactor, pile . Physics. an apparatus in which a nuclear-fission chain reaction can be initiated, sustained, and controlled, for generating heat or producing useful radiation.
5. Chem. (esp. in industry) a large container, as a vat, for processes in which the substances involved undergo a chemical reaction.
[ 1885-90; 1940-45 for def. 4; REACT + -OR 2 ]