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A reusable building block for a user interface — like a LEGO piece that you can snap together with others to build a full page.
A function (or class) that accepts props and returns JSX describing a portion of the UI. Components encapsulate markup, styles, and behavior into composable, reusable units that React renders and updates efficiently.
React components are pure functions of props → ReactNode. The runtime manages a fiber tree where each fiber represents a component instance with its own state, effects, and priority lane. Components that return the same output for the same props can be memoized via React.memo, skipping subtree reconciliation.