Hydration
In plain terms
Attaching interactivity to server-rendered HTML once it reaches the browser.
Going deeper
With server-side rendering you ship ready-made HTML for speed, then JavaScript 'hydrates' it by wiring up event handlers and state so it becomes interactive. It blends fast first paint with rich behavior.
Under the hood
Mismatches between server and client HTML cause hydration errors; partial/streaming/island hydration reduces the JavaScript cost of making large pages interactive.
Examples
Next.js, Nuxt, Remix server-rendered pages; React's hydrateRoot.
Think of it like...
Like delivering a fully-built model house, then flipping the breaker so the lights, doors, and appliances actually work.