magnetron(n = noun.artifact) - a diode vacuum tube in which the flow of electrons from a central cathode to a cylindrical anode is controlled by crossed magnetic and electric fields; used mainly in microwave oscillators;
is a kind of electron tube, thermionic tube, thermionic vacuum tube, thermionic valve, tube, vacuum tube

   a diode vacuum tube in which the flow of electrons from a central cathode to a cylindrical anode is controlled by crossed magnetic and electric fields; used mainly in microwave oscillators. [WordNet 1.5]

magnetron, n. an electron tube for amplifying or generating microwaves, with the flow of electrons controlled by an external magnetic field.

Etymology:
MAGNET + -TRON