Shortened names are everywhere, from news reports to text messages. Many are formed from the first letters of longer names or phrases. In the usual distinction, whether we call them acronyms or initialisms depends on how we pronounce them.
An acronym is pronounced as a word. You read the letters together rather than saying each letter’s name.
- NASA stands for National Aeronautics and Space Administration. It is pronounced “NASS-uh.”
- NATO stands for North Atlantic Treaty Organization. It is pronounced “NAY-toh.”
An initialism is pronounced letter by letter. Each letter keeps its spoken name.
- FBI stands for Federal Bureau of Investigation. It is pronounced “eff-bee-eye.”
- BBC stands for British Broadcasting Corporation. It is pronounced “bee-bee-see.”
Capital letters alone do not reveal the difference: NASA and FBI are both written in capitals, but we say them differently. To choose a label, pay attention to the spoken form, not just the spelling.
There is an important qualification: some dictionaries also use acronym as a broader term covering both types. You may therefore see FBI called an acronym in general writing. That reflects a broader definition, rather than necessarily being a mistake.
If you want to make the pronunciation distinction clear, use acronym for the word-like form and initialism for the letter-by-letter form.