A backronym or bacronym is a type of acronym that begins as an ordinary word, and is later interpreted as an acronym.
The word “backronym” is a portmanteau of back and acronym, and was coined in 1983.  The term is also used for a new set of words put to an existing acronym (e.g. “advanced research projects agency” → “arpa” → “address routing and parameters area”).
An acronym is a word created from the initial letters of a phrase: for example, Random Access Memory becomes RAM, pronounced as the word “ram”.
Reference: Wikipedia’s definition of Backronym, viewed 10 September 2006