Design system
A design system is a documented set of reusable components, patterns, and rules that keeps a product visually and behaviourally consistent.
In full
A design system is a shared library of components and the rules governing their use: typography scales, colour tokens, spacing units, buttons, form fields, cards, navigation patterns, and the guidance describing when each applies. It exists in both design tools and code, so that what is designed and what is built stay aligned. The purpose is consistency at scale. Without one, a site accumulates six subtly different button styles and four heading sizes as different people add pages over time, and the accumulated drift reads as carelessness.
Why it matters
It makes adding new pages faster and keeps them coherent. It also makes accessibility decisions once, at the component level, rather than relitigating them on every page.