NASA (pronounced like a word - you don't say "ehn eh ess eh").
It's acro- (height) -nym (word) - a word that exists on top of / above other words.
In contrast "NIH" is not an acronym because it isn't pronounced or read as a word. It's appropriate to say, "'NIH' is an abbreviation" or "'NIH' is an initialism". But saying "'NIH' is an acronym" is wrong!