thermionic emission vs emission

thermionic emission and emission both are nouns.

Word	Noun	Adjective	Verb	Adverb
thermionic emission	Yes	No	No	No
emission	Yes	No	No	No

As nouns, emission is a hypernym of thermionic emission; that is, emission is a word with a broader meaning than thermionic emission:

thermionic emission (noun)	emission (noun)
the emission of electrons from very hot substances	the act of emitting; causing to flow forth
	the occurrence of a flow of water (as from a pipe)
	the release of electrons from parent atoms
	any of several bodily processes by which substances go out of the body
	a substance that is emitted or released