A sign (♯, ♭, ♮) indicating a momentary departure from the key signature by raising or lowering a note a semitone, respectively called sharp, flat and natural (which cancels a previous sharp or flat.The term can also indicate the note raised or lowered. Also found as a double sharp and double flat.
Accidentals come in upward (sharp, ♯) and downward (flat, ♭) variations. If a scale degree that would normally be sharp or flat (e.g. f♯ in the D-major scale) is used as a natural (f), then this is also an accidental, indicated by the neutralizing sign ♮. That sign may also cancel a previously given accidental, but cancelling is only necessary within a measure: each bar line resets everything to the key signature given at the start of each stave.
There also exist special symbols for indicating double-sharp (𝄪) and double-flat (𝄫), i.e. a note change by a two semitones (typically used only, if the first modification was already provided by the key signature; a double-accidental is enharmonically equivalent to a full tone step), as well as microtonal extensions.