A glancing blow (originally known as a phantom hit) is the result of a hitbox just barely touching another fighter. It originated from Super Smash Bros. Melee and returned in Super Smash Bros. Brawl and Super Smash Bros. for Nintendo 3DS and Wii U. Glancing blows occur when an attack only just hits a target, and are usually accidental. In terms of the game's physics, the edges of the attack's collision bubble and the target's collision bubble are tangential (barely touching), as opposed to a normal hit, in which the collision bubbles overlap. Almost any attack has the potential to be a glancing blow, with the exceptions of attacks that cannot barely nick the target (such as homing attacks).
In Melee, glancing blows were commonly called "phantom hits", as they weren't given an official name until Brawl. Such hits would deal half damage, but deal no hitstun or knockback to the opponent.
In Brawl, glancing blows make a very quiet "chik" sound and produce small orange sparks; the attack doesn't deal hitstun, damage or knockback to the opponent.
Glancing blows are extremely rare and difficult to intentionally achieve, simply because the required circumstances (tangential collision bubbles) are correspondingly rare. As such, they are generally seen as an accident. However, some characters are set up for a glancing blow when in the Practice Stage before a Wi-Fi Match: Jigglypuff starts exactly the right distance away from the Sandbag to achieve a glancing blow with the first hit of its standard A 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 second Down tilt will result in a glancing blow. Additionally, when characters with attacks that have super armor frames use these attacks and activate said frames, they take hits as glancing blows.