Smooth landing (also known as lag cancelling, l-cancelling, or z-cancelling) is a technique that, when used, cuts all landing lag of an aerial in half (rounded up, meaning a move with 11 frames of landing lag would have 6 when smooth landed). Only regular aerials can be smooth landed, meaning special moves or grab aerials cannot.
A smooth landing is executed by pressing the designated button while an aerial is active. If the aerial is still active while the user hits the ground, the landing lag will be cut in half. If an aerial turns into a grounded attack when it hits the ground, smooth landing has no effect.