“Most readers probably know that an acronym is an invented word made up of the initial letters or syllables of other words, like NASA or NATO. Fewer probably know that an initialism is a type of acronym that cannot be pronounced as a word, but must be read letter-by-letter, like FBI or UCLA.”

Now muttering “CD-ROM” to myself…

Original TIL Source: a Patreon episode of The History of English Podcast