thermal emission vs thermionic emission

thermal emission and thermionic emission both are nouns.

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

As nouns, thermal emission and thermionic emission are synonyms defined as:

thermal emission and thermionic emission: the emission of electrons from very hot substances

thermal emission (noun)	thermionic emission (noun)
the emission of electrons from very hot substances	the emission of electrons from very hot substances