Nomenclature
The term acronym is the name for a word from the first letters of each word in a series of words (such as sonar, created from sound navigation and ranging). Attestations for "Akronym" in German are known from 1921, and for "acronym" in English from 1940. While the word abbreviation refers to any shortened form of a word or a phrase, some have used initialism or alphabetism to refer to an abbreviation formed simply from, and used simply as, a string of initials.
Although the term acronym is widely used to describe any abbreviation formed from initial letters, some dictionaries define acronym to mean "a word" in its original sense, while some others include additional senses attributing to acronym the same meaning as that of initialism. The distinction, when made, hinges on whether the abbreviation is pronounced as a word, or as a string of letters. In such cases, examples found in dictionaries include NATO (/ˈneɪtoʊ/), scuba (/ˈskuːbə/), and radar (/ˈreɪdɑr/) for acronyms, and FBI (/ˌɛfˌbiːˈaɪ/) and HTML (/ˌeɪtʃˌtiːˌɛmˈɛl/) for initialism. In the rest of this Wikipedia article, this distinction is not made.
There is no agreement on what to call abbreviations whose pronunciation involves the combination of letter names and words, such as JPEG (/ˈdʒeɪpɛɡ/) and MS-DOS (/ˌɛmɛsˈdɒs/).
There is also some disagreement as to what to call abbreviations that some speakers pronounce as letters and others pronounce as a word. For example, the terms URL and IRA can be pronounced as individual letters: /ˌjuːˌɑrˈɛl/ and /ˌaɪˌɑrˈeɪ/, respectively; or as a single word: /ˈɜrl/ and /ˈaɪərə/, respectively. Such constructions, however—regardless of how they are pronounced—if formed from initials, may be identified as initialisms without controversy.
The spelled-out form of an acronym or initialism (that is, what it stands for) is called its expansion.
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