meltdown vs overheating

meltdown and overheating both are nouns.

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

As nouns, overheating is a hypernym of meltdown; that is, overheating is a word with a broader meaning than meltdown:

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

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