an embellishing note usually written in smaller size
A short grace note (theoretically taking no time at all), occurring on the beat occupied by the main note to which it is prefixed, one scale-step higher or lower than that main note. (Sometimes equivalent, therefore, to a short appoggiatura, but in Baroque music interpreted differently and more strictly.) Written as a note lighter in appearance, typically a quaver (eighth note), with an oblique stroke through the stem.
This term comes from the Italian word acciaccatura, which is derived from the verb acciaccare, meaning "to crush." The original sense of crushing likely refers to the quick, staccato effect produced by the musical note.