systems
Composability
The capacity of small components to combine and nest into larger structures with capabilities neither piece had alone. Portability is a side-effect of doing this well – not the definition.
Think of Lego. A 2×4 brick isn’t valuable because you can hand it to a friend and it’s still a brick – it’s valuable because it snaps onto other bricks to build something neither brick could be alone. That snapping-together capability is composability. A wheel-brick, a window-brick and a flat-brick combine predictably into a car or a house because they share a connection interface, not because any one brick is impressive.
The mechanism underneath is reference, not copy: a search form composed from a label, an input and a button doesn’t contain duplicates of them – it holds a nesting relationship, so when the button component changes, every form built from it updates automatically. If composability meant “copy-paste portable between projects”, a design system’s value would just be “components I can drag somewhere else” – which describes a component library, a much smaller thing. A well-composed piece usually is easier to reuse elsewhere, precisely because it was built with a clean, minimal interface – but that ease is downstream of good composition, not what the word means.