A frame is the primary unit of time in all video games. Every frame, the game reads controller input, performs calculations, and renders an image on the screen. All Smash games run at 60 frames per second (60 FPS), meaning one frame is 1/60th of a second, or roughly 17 milliseconds. The amount of frames the game runs at per second is referred to as the "framerate" of the game.
The vast majority of timing-based elements in most fighting games, including the Smash Bros. series, are measured in frames. For example, Mario's forward tilt in Brawl comes out on frame 5, meaning it takes about 0.08333 seconds for the first hitboxes to appear upon inputting the attack. Players use many terms to refer to particular frames of an attack, such as invincibility frames. When invincibility frames are active, the character cannot be damaged when hit on a certain area. For example, Bowser has invincibility on his legs for part of his forward smash, and on his shell for part of his up smash.
Situations can occur where the game is not capable of finishing all its calculations within one frame, resulting in a delay while it finishes up. A notable example is on Fountain of Dreams in both Melee and Ultimate; many characters using special-effect-heavy attacks at once combined with the reflective floor can cause the game to stutter and lag as it fails to work fast enough to keep the framerate at its normal framerate. The disruption in framerate interferes with patterns that expect players to know exact frame times, as due to the varying framerate each frame does not always equal 1/60th of a second. This is one of the reasons the stage is banned in doubles in Melee and on all modes in Ultimate, as even minor lag can throw off experienced players.
Many mechanics are restricted to integer numbers of frames. For example, most hitboxes take their current position and their position one frame ago into account in order to stretch between the intervening space, so a fast-moving attack cannot pass through a target without damaging it. As another example, time-slowing effects in Smash 64 and Melee reduce the rendering framerate as well as the physics framerate, so using training mode to reduce game speed to 1/4x results in a noticeable drop in framerate. However, other elements utilize the concept of subframes, which allows decimal numbers of frames. For example, weight-based throws can animate at varying speeds, resulting in each drawn frame showing a subframe of each animation (such as frame 10.56). Starting with Brawl, this method is also used for rendering slowed-down gameplay, resulting in a consistent framerate at the slow speeds of training mode and when slow motion effects like that of the Timer are active.
In Super Smash Bros. for Nintendo 3DS, the graphics of Assist Trophies and Poké Ball Pokémon are rendered at 30 FPS, but their position, actions, and physics interactions are still calculated 60 times per second. This technique avoids taxing the 3DS's graphical processor too heavily.