in keyboard music of the Baroque period, a short grace note sounded together with the principal note or chord, but quickly released
Origin:
It < acciaccare, to crush < accia, ax < L ascia: see ax
See acciaccatura in American Heritage Dictionary 4
noun Music
An ornament note that is one half step or one whole step below a principal note and is sounded at the same time as the principal note, adding dissonance to a harmony.