accidental
theoryak-sih-DEN-tul
A sharp, flat, or natural sign that alters a note from the prevailing key signature.
An accidental is any symbol that changes a note's pitch from what the key signature indicates. Sharps raise a note by a semitone, flats lower it, and naturals cancel a previous sharp or flat. An accidental applies to all subsequent occurrences of that note within the same measure, then automatically expires at the next bar line.
Double sharps (𝄪) and double flats (𝄫) raise or lower a note by two semitones and appear in more complex harmonic contexts. Courtesy accidentals (also called cautionary accidentals) are placed as reminders even when not strictly necessary — they clarify the performer's reading, especially after bar lines or in passages with many chromatic alterations.
The tradition of courtesy accidentals — reminding performers of notes they should already know — exists because sight-reading under pressure makes even obvious things easy to miss.