Definition and background:
A short grace note, typically notated with a slash through the stem or flag that is a non-harmonic note sounded at the same time as the primary harmonic note or notes and either quickly resolves to the main note or is immediately released. The effect is that of dissonant notes being "crushed" together. This is also known as a short appoggiatura or crushed note. This ornament was common in 17th and 18th century keyboard performance.Also [Eng.] Short Appoggiatura; [Eng.] Crushed note; [Fr.] Pinc' 'touff' ; [Ger.] Kurzer Vorschlag or [Ger.] Zusammenschlag; [It.] acciaccatura.Written Notation Effect of Ornament OR' crushing; i.e., a very fast grace note that is "crushed" against the note that follows and takes up no value in the measureThe acciaccatura is of very brief duration, as though brushed on the way to the principal note, which receives virtually all of its notated duration.
For more information on the music term "Acciaccatura" check out these other resources:
Wikipedia - Glossary of Musical Terminology
Virginia Tech Multimedia Music Dictionary
ORB -- Medieval Music Glossary