Definition: A sign used in musical notation to indicate a chromatic alteration of a note. The standard signs are the sharp (#), flat (b), natural, double sharp, and double flat.
The Savvy Take: Always check the “blast radius” of an accidental. Remember that an accidental carries through the entire measure for that specific octave, but resets at the bar line. Missing this rule is the #1 cause of “wrong notes” in sight-reading.