The buffer is a mechanic in Super Smash Bros. Brawl, Super Smash Bros. 4, and Super Smash Bros. Ultimate which allows players to successfully input actions before they can actually be executed, causing the inputs to be carried out the first frame possible as soon at the first move or animation is finished. This allows players to perform techniques which would otherwise require frame-perfect precision, and can be used in combos to minimize the opponent's opportunity to escape from hitstun.
There are negative effects to buffered inputs. Buffering may lead to unintentional attacks that carry out (e.g. attempting to buffer a jab nearby the ledge while shielding, which would result in using a neutral aerial off the stage). This kind of situation can be deadly to characters with laggy aerials, such as Ike, whose recoveries do not provide enough vertical distance to recover after the attack ends. When playing online, the frame delay introduced to stabilize matches can affect buffering windows. When combined with possible Wi-Fi lag caused by a poor connection, unintentional inputs can become significantly more common in online play.
Smash 64 does not buffer inputs at all, and Melee does not use a buffer under normal circumstances. This is often cited as a reason for greater skill and precision requirements to perform combos at any level of play. In Melee, it is possible to buffer a shield, and thus possible to shield buffer some evasive options by holding the C-stick together with a shield input. The character will then proceed to jump (if held upwards), spot dodge (if held downwards) or roll (if held sideways) at the first instant possible. There is also a grace period of 3 frames in which one can fast fall after the peak of any jump.
Whenever a move is used in Brawl or Smash 4, a window of 10 frames is created at the end of every move and animation. Any input that is buffered during this time will be activated on the first frame possible. In Smash 4, it is not possible to buffer the following; any ledge option using the control stick, grab aerials, airborne item catches after an air dodge, Z-dropping items after an air dodge, shield after dropping the input during the 10 frame period, and all down special moves during soft or hard landing frames. There is also a hierarchy of moves that will come out when multiple inputs are pressed at the same time; special moves have the highest priority, followed by shields, attacks, jumps, and stick inputs, in that order. This hierarchy is the reason why buffering both a control stick input and a button input at the same time is impossible and thus, a common problem when trying to buffer moves like dash attack or dash grab out of shield. For ledge options, ledge jumping has the highest priority, followed by ledge attacking and ledge rolling.
While Ultimate retains the buffering system from Brawl and Smash 4 (with the window reduced to 9 frames), it also introduces a second buffering system: holding an input through the end of a previous move's animation will now cause the held input to be buffered. The input can be made as early as desired, but must be held until the standard 9 frame buffer window is reached. For example, holding the forward smash input the entire time after using a recovery onstage will result in it charging when the character lands. This can also cause other negative side effects while bringing its own advantages, such as making buffered airdodges more likely, but also making the online mode have more consistency in inputs.