An acronym (pronounced AK-ruh-nihm, from Greek acro- in the sense of extreme or tip and onyma or name) is an abbreviation of several words in such a way that the abbreviation itself forms a pronounceable word. The word may already exist or it can be a new word. Webster's cites snafu and radar, two terms of World War Two vintage, as examples of acronyms that were created.
According to the strictest definition of an acronym, only abbreviations that are pronounced as words qualify. So by these standards, for example, COBOL is an acronym because it's pronounced as a word but WHO (World Health Organization) is not an acronym because the letters in the abbreviation are pronounced individually. However, opinions differ on what constitutes an acronym: Merriam-Webster, for example, says that an acronym is just "a word formed from the initial letters of a multi-word name."
Frequently, acronyms are formed that use existing words (and sometimes the acronym is invented first and the phrase name represented is designed to fit the acronym). Here are some examples of acronyms that use existing words:
Abbreviations that use the first letter of each word in a phrase are sometimes referred to as initialisms. Initialisms can be but are not always acronyms. AT&T, BT, CBS, CNN, IBM, and NBC are initialisms that are not acronyms. Many acronym lists you'll see are really lists of acronyms and initialisms or just lists of abbreviations. (Note that abbreviations include shortened words like "esp." for "especially" as well as shortened phrases.)
- An abbreviation is a shortening of a word or a phrase.
- An acronym is an abbreviation that forms a word.
- An initialism is an abbreviation that uses the first letter of each word in the phrase (thus, some but not all initialisms are acronyms).
- An acronym so familiar that no one remembers what it stands for is called an anacronym (For example, few people know that COBOL stands for Common Business Oriented Language.)
- An acronym in which one of the letters stands for the actual word abbreviated therein is called a recursive acronym. (For example, VISA is said to stand for VISA International Service Association.)
- 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.)