acciaccatura – crushing; i.e., a very fast grace note that is "crushed" against the note that follows and takes up no value in the measure.
From the Italian word acciaccare, "to crush"; (pronounced approximately [əˌtʃækə̆ˈtuˑɾə]). The acciaccatura (sometimes called short appoggiatura) is perhaps best thought of as a shorter, less melodically significant, variant of the long appoggiatura, where the delay of the principal note is scarcely perceptible - theoretically subtracting no time at all. It is written using a grace note (often a quaver, or eighth note), with an oblique stroke through the stem:
The exact interpretation of this will vary according to the tempo of the piece, but the following is possible:
Whether the note should be played before or on the beat is largely a question of taste and performance practice. Exceptionally, the acciaccatura may be notated in the bar preceding the note to which it is attached, showing that it is to be played before the beat. (This guide to practice is unfortunately not available, of course, if the principal note does not fall at the beginning of the measure.)
The implication also varies with the composer and the period. For example, Mozart's and Haydn's long appoggiaturas are –to the eye– indistinguishable from Prokofiev's and Moussorgsky's before-the-beat acciaccaturas.
In some cases on instruments that permit it, such as the piano, the acciaccatura is sounded simultaneously with the principal note, and then immediately released.