noun (used with a singular verb)
the branch of physics that deals with thermionic phenomena.
Origin of thermionics
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
(functioning as singular) the branch of electronics concerned with the emission of electrons by hot bodies and with devices based on this effect, esp the study and design of thermionic valves
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012