overheating vs meltdown

overheating and meltdown both are nouns.

Word	Noun	Adjective	Verb	Adverb
overheating	Yes	No	No	No
meltdown	Yes	No	No	No

As nouns, meltdown is a hyponym of overheating; that is, meltdown is a word with a more specific, narrower meaning than overheating:

overheating: excessive heating
meltdown: severe overheating of the core of a nuclear reactor resulting in the core melting and radiation escaping

Other hyponyms of overheating include nuclear meltdown.

overheating (noun)	meltdown (noun)
excessive heating	a disaster comparable to a nuclear meltdown
	severe overheating of the core of a nuclear reactor resulting in the core melting and radiation escaping