commence vs plunge

commence is not a noun while plunge is a noun.

Word	Noun	Adjective	Verb	Adverb
commence	No	No	Yes	No
plunge	Yes	No	Yes	No

As verbs, plunge is a hyponym of commence; that is, plunge is a word with a more specific, narrower meaning than commence:

commence (noun)	plunge (noun)
	a brief swim in water
	a steep and rapid fall

commence (verb)	plunge (verb)
take the first step or steps in carrying out an action	begin with vigor
set in motion, cause to start	devote (oneself) fully to
get off the ground	cause to be immersed
	immerse briefly into a liquid so as to wet, coat, or saturate
	thrust or throw into
	fall abruptly
	drop steeply
	dash violently or with great speed or impetuosity