Frame
A frame is a single, still image displayed by the computer, video hardware, or software application and part of a larger sequence of images that make up a video or computer game. A frame is not necessarily a unit of time, though it can be used to measure time to an extent. Every frame, the game reads controller input, performs calculations, and renders an image on the screen.
Overview
The vast majority of timing-based elements in most video games, including the Smash Bros. series, are measured in frames. This is primarily done because using clean integers that frames are measured in is easier to comprehend than in fractions of a second. The amount of frames the game runs at per second is referred to as the "framerate" of the game. For example, Mario's forward tilt in Brawl comes out on frame 5 in a game that runs at 60 frames per second, meaning it takes about 0.08333 seconds for the first hitboxes to appear upon inputting the attack. Also, some games utilize the concept of subframes, or fractions of a frame (such as frame 10.56). The primary usage of subframes is to make a hyper-specific aspect of gameplay function as intended or keep said aspects working under any kind of speed or time manipulation in-game.
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. The tangible result of this delay is often referred to as "lag." The disruption in framerate interferes with techniques that involve a meaningful amount of precision, as a differing framerate for any amount of time can throw off timing, the severity of which changing at random. This is why lag is generally met with disdain when discovered and is often patched out in modern games that are able to.
In the Super Smash Bros. series
All Smash games run at 60 frames per second (60 FPS), meaning one frame lasts 1/60th of a second, or 16.67 milliseconds.
A notable example of lag caused the game having issues making all necessary calculations is on Fountain of Dreams in both Melee and Ultimate; characters and items using special-effect-heavy animations combined with the reflective floor can cause the framerate to noticeably decrease as the game engine tries and fails to render the image in its proper resolution within a frame. This is one of the reasons why 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 subframes. For example, weight-based throws can animate at varying speeds, resulting in each drawn frame showing a subframe of each animation. 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 (half the amount of FPS the overall game runs at), 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.
