initialism

An abbreviation consisting of the first letter or letters of words in a phrase (for example, IRS for Internal Revenue Service), syllables or components of a word (TNT for trinitrotoluene), or a combination of words and syllables (ESP for extrasensory perception) and pronounced by spelling out the letters one by one rather than as a solid word.

A term formed from the initial letter or letters of several words or parts of words, but which is itself pronounced letter by letter.