Why is NASA an acronym but FBI isn't?

Say 'NASA' out loud and it comes out as one word. Say 'FBI' out loud and it comes out as three letters: eff, bee, eye. That difference is exactly why some language authorities insist only one of them truly deserves the word 'acronym'.

In the strict, narrow sense, an acronym is a string of initial letters pronounced together as a single word, like NASA, NATO, or laser, which started life as 'light amplification by stimulated emission of radiation'. Abbreviations that are instead spelled out letter by letter, like FBI, DVD or VHF, get a different technical name: initialisms. But everyday usage rarely respects that line. People call both kinds 'acronyms' all the time, and dictionaries have been fighting about it for decades.

The word itself is surprisingly young. It was built from Greek roots meaning 'tip' and 'name', apparently coined in German around 1921, and it first shows up in English in 1940, in a translated novel. From the start, both the narrow sense and the broader one existed side by side: in 2010, the Duke University researcher Stephen Goranson found a 1940 reference to the abbreviation PGN, pronounced letter by letter as 'pee-gee-enn', already being called an acronym.

Style guides still take sides. Bryson's Dictionary of Troublesome Words insists that something like IBM or NFL is 'just an abbreviation', not a true acronym. But dictionaries moved the other way: Merriam-Webster began defending the broader sense in 1994, and by 2011 the Oxford English Dictionary had added it too, now listing it as the word's very first meaning, ahead of the stricter one.