A three-letter acronym, three-letter abbreviation, or TLA is an acronym, abbreviation, alphabetism or initialism consisting of three letters. These are usually the initial letters of words of the phrase abbreviated, and are written in capital letters (upper case): three-letter abbreviations such as "etc." or "Mrs." would not be described as a three-letter acronym.
Most three-letter abbreviations are initialisms (i.e., all the letters are pronounced as letters, as in LSD), and very few fit the narrow definition of acronym (which requires it to be pronounced as a single word, as in DOS, which is unusual in three-letter abbreviations). See acronym and initialism for discussion of the difference.
Nevertheless, three-letter acronym is a recognised phrase, often abbreviated to TLA. A series of lists of TLAs lists all TLAs which are included in Wikipedia. These lists also include abbreviations comprising two letters followed by a single digit, which are not strictly three-letter acronyms.
TLA has the recursive feature that TLA is its own TLA.
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