abbreviation

shortened form of a word, phrase, or name where the abbreviation has not become part of the language

This includes initialisms and acronyms where:

initialisms are shortened forms of a name or phrase made from the initial letters of words or syllables contained in that name or phrase

Not defined in all languages.

SNCF is a French initialism that contains the initial letters of the Société Nationale des Chemins de Fer, the French national railroad.

ESP is an initialism for extrasensory perception.
acronyms are abbreviated forms made from the initial letters or parts of other words (in a name or phrase) which may be pronounced as a word

NOAA is an acronym made from the initial letters of the National Oceanic and Atmospheric Administration in the United States.

Some companies have adopted what used to be an initialism as their company name. In these cases, the new name of the company is the letters (for example, Ecma) and the word is no longer considered an abbreviation.