An acronym is a word or name formed from the initial components of a longer name or phrase.
An acronym is an abbreviation of a phrase that usually consists of the initial letter of each word in all caps with no punctuation.
For some, initialism, or alphabetism, connotes this general meaning, and acronym is a subset—pronounced as a word rather than as letters. In this sense, NASA is an acronym but USA is not.
The broader sense of acronym, ignoring pronunciation, is its original meaning and in common use. Dictionary and style-guide editors dispute whether the term acronym can be legitimately applied to abbreviations which are not pronounced as words, nor do they agree on acronym spacing, casing, and punctuation.
The phrase that the acronym stands for is called its expansion. The meaning of an acronym includes both its expansion and the meaning of its expansion.