acciaccatura explained
A very short ornamental note played as quickly as possible before the main note, taking no perceptible time
In Depth
The acciaccatura (from the Italian acciaccare, to crush) is printed as a small note with a slash through its stem. Unlike the appoggiatura, it is played before the beat (or crushed against the main note on the beat) and released immediately. The effect is a sharp, percussive snap of dissonance. In keyboard music, both notes may be struck simultaneously with the grace note released instantly. The acciaccatura adds rhythmic bite and expressive colour without significantly delaying the main melody.
Scarlatti's keyboard sonatas contain hundreds of acciaccaturas that imitate the rasgueado strumming technique of Spanish guitar.