Three-letter acronym

A three-letter acronym, three-letter abbreviation, or TLA is an abbreviation, specifically an acronym, alphabetism, or initialism, consisting of three letters. These are usually the initial letters of the words of the phrase abbreviated, and are written in capital letters (upper case); three-letter abbreviations such as etc. and Mrs. are not three-letter acronyms, but "TLA" itself is a TLA (an example of a self-referencing definition).

Most three-letter abbreviations are initialisms: all the letters are pronounced as the names of letters, as in APA /ˌeɪpiːˈeɪ/ AY-pee-AY. Some are acronyms pronounced as a word; computed axial tomography, CAT, is almost always pronounced as the animal's name in "CAT scan".

The exact phrase three-letter acronym appeared in the literature in 1975. Three-letter acronyms were used as mnemonics in biological sciences, and their practical advantage was promoted by Weber in 1982. They are used in many other fields, but the term TLA is particularly associated with computing. The specific generation of three-letter acronyms in computing was mentioned in a JPL report of 1982.

In 1980, the manual for the Sinclair ZX81 home computer used and explained TLA. In 1988, in a paper titled "On the cruelty of really teaching computer science", eminent computer scientist Edsger W. Dijkstra wrote "Because no endeavour is respectable these days without a TLA ..." By 1992 it was in a Microsoft handbook.

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