Acronyms are new words made up from initials. ATM is an initialism which is where each letter is said individually.
Seems like a simple question. Of course ATM is an acronym, right? Actually wrong.
An acronym is a new word that is created using the initials from a series of words, but when you pronounce each of the letters individually, that’s something else – an initialism.
An abbreviation formed from the initial letters of other words and pronounced as a word (e.g. ASCII, NASA).
An abbreviation consisting of initial letters pronounced separately (e.g. BBC).
According to Mirriam Webster – the appearance of the word initialism precedes the word acronym, by about half a century. They do however acknowledge that while some grammarians feel strongly about the distinction, most people use the word acronym to describe both forms.
A website called Daily Writing Tips points out that the key difference between using an initialism or an acronym is whether you use the word the before it. For example, you would always say “the FBI”, but you generally wouldn’t say “the NASA”.