A crouch cancel (abbreviated CC) is a technique in both Super Smash Bros. and Super Smash Bros. Melee used to reduce the knockback of an attack on the character. By holding down on the control stick before getting hit by an attack, the player will not be moved as far as normal, often stopping their movement completely at lower Damage Percentages. Crouch cancelling has the added effect of lowering the trajectory your character is sent after an attack.
Crouch cancel counters
Many characters use crouch cancelling to perform a crouch cancel counter (CCC), where the player performs a strong counterattack after crouch cancelling an incoming attack. Peach and Samus are well-known abusers of crouch cancel counters; both characters can crouch cancel most attacks and perform a down smash before their opponent recovers (SSBM only).