An acronym is a word or name consisting of parts of the full name's words. Acronyms are usually formed from the initial letters of words, as in NATO (North Atlantic Treaty Organization), but sometimes use syllables, as in Benelux (short for Belgium, the Netherlands, and Luxembourg), NAPOCOR (National Power Corporation), and TRANSCO (National Transmission Corporation). They can also be a mixture, as in radar (Radio Detection And Ranging) and MIDAS (Missile Defense Alarm System).

Acronyms can be pronounced as words, like NASA and UNESCO; as individual letters, like CIA, TNT, NPC, BLM, and ATM; or as both letters and words, like JPEG (JAY-peg), CSIS (SEE-sis), and IUPAC (I-U-pak). Some are not universally pronounced one way or the other and it depends on the speaker's preference or the context in which it is being used, such as SQL (either "sequel" or "ess-cue-el").

The broader sense of acronym—the meaning of which includes terms pronounced as individual letters— is sometimes criticized, but that is the term's original meaning, and is still in common use. Dictionary and style-guide editors are not in universal agreement on the naming for such abbreviations, and it is a matter of some dispute whether the term acronym can be legitimately applied to abbreviations which are not pronounced "as words", nor do these language authorities agree on the correct use of spacing, casing, and punctuation.

Abbreviations formed from a string of initials and usually pronounced as individual letters are sometimes more specifically called initialisms or alphabetisms; examples are FBI from Federal Bureau of Investigation, ABS-CBN from Alto Broadcasting System – Chronicle Broadcasting Network, GMA from Global Media Arts, NPC from National Power Corporation, NGCP from National Grid Corporation of the Philippines, and e.g. from Latin exempli gratia.