We are aware that an accidental is a sign which changes the pitch of a note usually stated in the key signature, like a natural sign before a C in key D makes that into C♮, etc. Sometimes even the key signature is said to contain accidentals (I'm sceptical).
The first two notes that needed them were B and F, and the Germans cunningly got round the B by introducing another letter. F didn't fare so well, and F♯ was born, followed by other re-incarnations.
However - it's the etymology that's in dispute for this question. Whenever accidentals are used, they're never used accidentally! It's always on purpose! So the word itself is a misnomer, and would be confusing if we didn't know any better. It probably confounds beginners, too. So, where and why, and by whom was it coined. And is there a more concise word instead?
I've come across inflection in the Oxford Companion to Music, which seema to cover all the 'accidentals'.