A glancing blow (チップ chippu, chip), known in Super Smash Bros. Melee as a phantom hit, is the result of a hitbox making insufficient contact with a damageable object - that is, the attack's hitbox does not connect deep enough into the target's hurtbox to be considered a solid hit. Glancing blows are rather rare and almost always accidental.
In Melee, glancing blows were commonly called "phantom hits", as they weren't given an official name until Super Smash Bros. Brawl. Such hits occur when a hitbox connects less than 0.01 units through a hurtbox and do half damage with no knockback or flinching, with the attacker not subjected to any freeze frames (but the defender is). There is nothing indicating that a glancing blow was made and the damage dealt by a phantom hit is not applied until the defender's freeze frames run out, allowing the move a chance to connect fully on a later freeze frame. A phantom hit does not count as hitting the target, so this behaviour can cause the move to hit twice (once with the phantom hit and again with the proper hit) if full contact is made after freeze frames have ended and phantom hit damage has been applied. It is possible to SDI during the hitlag, but only if already in a knockback action from being hit with something else. Phantom hits can never be ASDIed. It is unknown whether they count in the stale moves queue, though it is likely they do not.
In Brawl, glancing blows now occur when hitboxes connect less than 0.1 units through a hurtbox, happening far more easily than in Melee, and make a very quiet "chik" sound as well as producing small orange sparks. They also deal no damage at all and do not produce freeze frames for the defender; this removes the double-hit effects that Melee had. As the "chik" and sparks occur on the first frame of the hit, it is now possible to see an attack have a glancing blow on one frame and a solid connection on the next (as opposed to Melee, where this was undetectable at game speed).
Glancing blows are rare and difficult to intentionally achieve, simply because the required circumstances (a hitbox and a hurtbox being an extremely precise distance from one another) are correspondingly rare. As such, they are generally seen as an unlucky accident. However, some characters are set up for a glancing blow when on the Online Practice Stage; Jigglypuff starts exactly the right distance away from the Sandbag to achieve a glancing blow with the first hit of its standard attack (jab) or the third "hit" of Sing; and one of Kirby's down tilts will put him exactly the right distance away from the Sandbag so that a repeat will result in a glancing blow. Using Luigi's down tilt will result in a glancing blow as well.
Despite extreme difficulty in forcing glancing blows, there exist multiple Tool Assisted Speedrun videos of Melee that are composed almost entirely of glancing blows; such videos are generally used to demonstrate one's skill at TAS.
Glancing blows are believed to not exist in the original Super Smash Bros., due to a lack of evidence combined with how it uses cubes for attack physics instead of spheres. It is also believed that glancing blows only occur between hitboxes and hurtboxes, and not between hitboxes and reflectors or absorbers.