Crouching, often simply called Ducking, is a ground-based position achieved by holding down on the control stick when standing. Crouching shortens the character's damageable collision bubble and can be used to crouch cancel enemy attacks. Crouch canceling has no effect in SSBB.
SSBM's lowest crouching characters are Kirby, Jigglypuff, Pichu and Sheik.
SSBB's lowest crouching characters are Kirby and Jigglypuff.