Form & CompositionRevised
System
A set of distinct components governed by one shared rule across a larger whole.
In practice
A system is a set of distinct parts that all follow one shared rule, so they belong together across a larger whole. A design system for an app defines a few spacing values, colors and component rules that every screen uses. A set of airport pictograms with the same line weight, corner radius and grid is also a system: each sign differs, yet any new one can be drawn to fit.
Often confused with Structure
A system governs many distinct components with one rule; structure is the internal organization holding a single object's own parts together.
Try this
Draw four simple icons, a house, cup, star and phone, using one rule: the same stroke width and rounded corners.