The American Heritage® Dictionary of the English Language, 4th Edition
n. The physics of thermionic phenomena.
WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
n. the branch of electronics dealing with thermionic phenomena (especially thermionic vacuum tubes)
Word Usage
"He was awarded the Hughes Medal by the Royal Society (1920), especially for work on thermionics; elected President, Section A, of the British Association (1921) and President of the Physical"