Module
A single repeatable unit whose form determines the structure and proportions of a larger composition.
In practice
A module is one repeatable unit whose shape and size set the rules for a larger whole. When modules repeat, the whole inherits their proportions. Modular shelving built from identical cubes, a floor of hexagonal tiles and a web page made of equal cards all depend on the module chosen first. Change the module from a square to a long rectangle and the whole composition changes character.
Often confused with Pattern
A module is the single repeatable unit; a pattern is the larger motif that emerges when units are repeated according to a rule.
Try this
Cut eight identical paper shapes, arrange them three ways, and note how the module limits what is possible.