An acronym is an abbreviation, often composed of the initial letters of the words in a short phrase, that is treated as word (often, a piece of jargon or the proper name of an organization). For example, SAM for ''s''urface-to-''a''ir ''m''issile and NATO for the North Atlantic Treaty Organization. In its original meaning, acronyms were restricted to pronouncible abbreviations (what might be called true acronyms), though common usage permits calling unpronouncable abbreviations acronyms as well. Sometimes conjuntions and prepositions (such as and or to) contribute letters to make the acronym pronouncible, in contradiction to the normal English rule for abbreviations.
Often, an acronym will come into such wide use that people think of it as a word in itself, forget that it started out as an acronym, and write in in small letters. Examples include quasar (quasi-stellar radio source), laser (light amplification by stimulated emission of radiation) and radar (radio detection and ranging).
Non-pronouncible abbreviations formed from initials (such as IBM for International Business Machines) are sometimes called initialisms.
Some lists of acronyms in use:
A large list of acronyms may be found at http://www.acronymfinder.com/