An acronym is spoken as a word, while an initialism is read letter by letter, as in scuba and HTML.

The longer version

Whether a string of initials is said as one word or spelled out decides the category. Groups that form pronounceable syllables get spoken, so scuba reads as a word; groups that would be awkward to say are read letter by letter. The distinction is about the mouth, not about punctuation or capitalisation.

Some labels drift between the two. Groups that start as spelled-out abbreviations may acquire a spoken nickname, and a few are designed so the initials spell something memorable. Scientific naming leans on this heavily, since a pronounceable short form survives in speech far better than a list of letters, and many such words later become ordinary nouns.

Where this entry sits

This category gathers small surprises about the English language, from odd spelling rules to words that quietly hide their own history. Expect palindromes, punctuation oddities, borrowed terms, and the strange ways writing systems record spoken speech. The full list sits on the Words and Language page, where every entry is listed in order.