Atomic Design

Atomic design is a method, coined by Brad Frost, for building interfaces out of small reusable pieces that combine into larger ones: atoms into molecules into organisms, and on up to whole pages. Everything is made of the same basic parts arranged in different ways.

In practice it is the thinking behind a pattern library and most modern design systems. Rather than designing pages, you design components and the small elements inside them, then assemble them. This keeps a site consistent and makes it easier to change one piece everywhere at once.