when the letters of a word stand for something
Acronym Whereas an abbreviation may be any type of shortened form, such as words with the middle omitted (for example, Rd for road or Dr for Doctor), an acronym is a word formed from the first letter or first few letters of each word in a phrase (such as sonar, created from sound navigation and ranging). Attestations for Akronym in German are known from 1921, and for acronym in English from 1940.[1]