Crouch-canceling (abv CC) is the act of holding down on the control stick when hit by an attack to reduce knockback. Up to a certain percentage, this will prevent you from being sent flying, but you will slide back. CCing an upsmash will let you survive it much longer than normal. Also, a crouch cancel counter (ccc) is crouch canceling using a character's down-A attack. It is also useful when disrupting a shine combo.
Credits: CoSK