The angle of an attack, also known as trajectory, is what direction it sends opponents when it connects, such as horizontally, vertically, or diagonally. Many attacks have multiple hitboxes that possess slightly (or even significantly) different angles, so opponents being hit by different parts of one attack can be sent in different directions. Angle is a basic property of all hitboxes that has a major effect on an attack's effectiveness as a whole.
The angle of a hitbox is defined as a number of degrees and can span any integer number, where 0 equals "away from the attacker", 90 equals "straight up", 180 equals "towards the attacker", and 270 equals "straight down"; negative angles such as -90 can also be used, and can carry an additional meaning depending on the game.
Types of angles
Generally, no one angle is better than any other; two otherwise identical attacks that have differing angles will often have different purposes. One of the most dramatic of these comparisons is in Melee: despite Fox's and Falco's Reflectors having very similar properties, and both being central to their characters, they have completely different usages in competitive play. A character that lacks a type of move within one of these broad classes (for example, a vertical KO move) will often be at a disadvantage compared to a character that has at least one of every option.
Generally, the definition of "away" versus "towards" is based on where the attacker and the opponent are positioned, not the hitboxes and hurtboxes involved. In SSB and Melee, all attacks work this way, which results in some attacks being able to send opponents in unexpected directions; Fox's back air and Captain Falcon's Knee Smash in Melee are notable examples of cases where players intentionally try to land "reverse" hits for combo purposes. Likewise, throwing an item against an opponent too close to them might cause the hit to be registered from behind them, and thus launch them towards the attacker instead of away.
From Brawl onward, hitboxes can have a property known as an angle flipper set to them, which can determine whether the target is always set in the direction the attacker is facing (forward), or opposite to said direction (backward), among other variables. Otherwise, this direction is based on the attacker and target's positions as usual. Although angle flippers are relatively uncommon in Brawl, several moves in Smash 4 use them, namely various forward smashes always sending the target forward, and most back aerials always launching them backward (forward for characters whose back aerials turn them around). They are also present on all pummels and edge attacks, which launch the target forward. In Ultimate, angle flippers are even more widespread: all characters' entire neutral attacks, as well as almost all dash attacks, forward tilts, forward smashes, and forward aerials launch opponents forward, while all back aerials launch them backward (including those that did not in Smash 4); the only exceptions are King Dedede, Mewtwo, and Wii Fit Trainer's dash attacks, R.O.B.'s forward tilt and forward aerial, Luigi's forward smash and forward aerial, Olimar's forward aerial (except with Red Pikmin), and Wario's forward aerial.
Since angle flippers overall cause the behavior of a move's knockback to be more consistent, they tend to be beneficial for moves aimed on a specific direction. For example, forward tilts and forward smashes can ensure the opponent is sent in the desired direction and preserve the user's advantage state, instead of ending up in a position that is more difficult to capitalize on. On the other hand, they can be detrimental to certain moves with notable reach behind the user, such as Marth's forward aerial and Ike's forward smash, as the former's cannot be used for more unique followups if it hits from behind, while the latter is less effective for catching opponents near the edge attempting to jump over Ike. However, certain moves that hit around the user, more specifically multi-hitting ones, can benefit from angle flippers, allowing the user to more consistently determine the opponent's ending position after the final hit and either start and extend combos; notable examples of this are Bayonetta's Witch Twist, and Palutena's neutral aerial in Ultimate.
Certain angles are dynamic, changing the exact direction they launch based on additional factors.