RSC React Server Components
React components that run only on the server, are never shipped to the browser, and may fetch data directly instead of going through an API.
The difference from ordinary server rendering is that this is not about generating HTML once and then shipping all the code anyway. A server component never appears in the client bundle at all — its dependencies, say a heavy markdown library, do not travel with it. The output is streamed in a dedicated format that the client weaves into its component tree, so parts of the page can arrive progressively rather than waiting for everything. The boundary is marked with the "use client" directive, and only below it do state, effects and event handlers exist. The idea was presented in December 2020, became usable in the Next.js App Router in 2023, and stabilised in React 19.