an acronym or abbreviation formed from initial letters, specif., one pronounced using the letters' names rather than phonetically (Ex.: CD, DNA, UHF)
See initialism in American Heritage Dictionary 4
noun
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.