Unit

A unit is the generic measurement used by all 3D software to measure distances and define coordinates.

A unit's relationship to real-world measurements is not set in stone or precise; it is defined only by the scale of a game's content.

Units in Id Software and Valve engines are referred to as quake units (qu) or hammer units (hu), and are usually loosely analogous to roughly 1 inch, 0.75 inches, or 2-3 centimeters (see Dimensions).
Blender refers to units equaling meters by default, but this can be changed.
The glTF model standard (supported by Source 2's tools and most modern modeling software) requires units to equal meters, but not all software respects this.

See also