initialism

Acronyms and initialisms are abbreviations, such as NATO, laser, and IBM, that are formed using the initial letters of words or word parts in a phrase or name. Acronyms and initialisms are usually pronounced in a way that is distinct from that of the full forms for which they stand: as the names of the individual letters (as in IBM), as a word (as in NATO), or as a combination (as in IUPAC). Another term, alphabetism, is sometimes used to describe abbreviations pronounced as the names of letters.
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ACRONIMO COMPOSTO DALLE LETTERE INIZIALI. SIGLA