Real Dictionary

acronym
countable noun: An acronym is a word composed of the first letters of the words in a phrase, especially when this is used as a name. An example of an acronym is 'NATO', which is made up of the first letters of the 'North Atlantic Treaty Organization'.

acronym in American English
a word formed from the first (or first few) letters of a series of words, as radar, from radio detecting and ranging
noun: a word formed from the initial letters or groups of letters of words in a set phrase or series of words, as Wac from Women's Army Corps, OPEC from Organization of Petroleum Exporting Countries, or loran from long-range navigation

acronym in British English
noun: a pronounceable name made up of a series of initial letters or parts of words; for example, UNESCO for the United Nations Educational, Scientific, and Cultural Organization