roar vs thunder

roar and thunder both are nouns.

Word	Noun	Adjective	Verb	Adverb
roar	Yes	No	Yes	No
thunder	Yes	No	Yes	No

As nouns, roar and thunder are synonyms defined as:

Other synonyms of roar include boom, roaring.

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

Other hyponyms of roar include vroom, bawl, yawp.

As verbs, roar and thunder are synonyms defined as:

roar (noun)	thunder (noun)
a very loud utterance (like the sound of an animal)	street names for heroin
a deep prolonged loud noise	a deep prolonged loud noise
the sound made by a lion	a booming or crashing noise caused by air expanding along the path of a bolt of lightning

roar (verb)	thunder (verb)
laugh unrestrainedly and heartily	utter words loudly and forcefully
make a loud noise, as of wind, water, or vehicles	to make or produce a loud noise
emit long loud cries	move fast, noisily, and heavily
utter words loudly and forcefully	be the case that thunder is being heard
make a loud noise, as of animal
act or proceed in a riotous, turbulent, or disorderly way