Distance unit
A distance unit (often shortened to unit) refers to an arbitrary unit of measurement that calculates the in-game position and size of objects.
Overview
Many aspects of video games are reliant on distance to perform specific interactions, ranging from how a character moves, physic engine interactions, to how scenes are rendered. A common practice in 3D games is to correlate units to a real-world measurement (e.g. 1 unit = 1 meter), which is commonly done with games that are based on the real-world in order for level design, movement, physics, etc. to accurately/predictably be replicated.
In the original Super Smash Bros, Masahiro Sakurai claims that one unit is equal to 1/3cm.[1] Starting in Super Smash Bros. Melee and remaining unchanged in all games since, the unit size was redefined to be an estimated one decimeter, or 1 unit = 0.1 meters.[2] As a result, the "modern" unit is about 30 times the "original" unit, and units between all games can be compared fairly easily. Despite units being compared to real-world units, the size of something in Smash Bros. is generally based on feel and balance, rather than its actual size in its home series or real life.
Applications
Hitboxes and hurtboxes use units to define their position and size. The majority of character attributes are measured in "units per frame", such as jump height, walk speed, run speed, air speed, falling speed, and so on. Accelerative stats such as air acceleration and gravity measure in units/frame², and frictional stats such as traction and air friction use negative units/frame². Very few attributes use a different system, such as weight.
Super Smash Bros. Melee introduced the concept of glancing blows. A glancing blow is activated when a hitbox and an opponent's hurtbox overlaps by 0.01 units or less. Later games, beginning with Brawl, widen this range to 0.1 units.
The Training stage in Super Smash Bros. Ultimate has a background that uses a grid. This grid is intended to have each line be 1 unit apart, however this correlation was performed on the squares between the grid lines and ignoring the size of the grid lines themselves. The texture for the grid uses 15×15 pixel squares with 1 pixel wide grid lines, resulting in the grid lines being 1.06 (16/15) units apart.