backronym和apronym
An acronym in which the short form was original and words made up to stand for it afterwards is called a backronym. (For example, SOS was originally chosen as a distress signal because it lent itself well to Morse code. Long versions, including Save Our Ship and Save our Souls, came later.)
An acronym whose letters spell a word meaningful in the context of the term it stands for is called an apronym. (For example, BASIC, which stands for Beginner's All-purpose Symbolic Instruction Code, is a very simple programming language.)