An initialism is an abbreviation formed from the first letters of a series of words, where each letter is pronounced separately. When you say “FBI” as “eff-bee-eye” instead of trying to pronounce it as a word, you're using an initialism. Other common initialisms include USA (you-ess-ay), DVD (dee-vee-dee), and ATM (ay-tee-emm).

Initialisms differ from acronyms, which are also formed from first letters but are pronounced as words themselves. NASA is an acronym because you say it like “nassah,” not “enn-ay-ess-ay.” SCUBA is an acronym (it stands for Self-Contained Underwater Breathing Apparatus), while FBI is an initialism.

The distinction matters when you're writing: you put “a” before consonant sounds and “an” before vowel sounds. You'd write “an FBI agent” because FBI starts with the “eff” sound, but “a NASA scientist” because NASA starts with the “nass” sound.

People create initialisms to save time and space. Saying “Federal Bureau of Investigation” every time would be exhausting, so “FBI” works perfectly. Initialisms are everywhere once you start noticing them: your school might use initialisms for clubs (like PTA for Parent-Teacher Association), scientists use them constantly (DNA, GPS), and you probably text with initialisms too (like LOL, BTW).