an ion or electron emitted by a substance at high temperature.
Etymology: THERMO- + ION thermionic adj. of or relating to electrons emitted from a substance at very high temperature.
Phrases and idioms:
thermionic emission the emission of electrons from a heated source. thermionic valve (US tube) a device giving a flow of thermionic electrons in one direction, used esp. in the rectification of a current and in radio reception.