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The environment where code actually executes: browser, Node.js, edge worker, serverless function, or mobile app.
Runtime determines what APIs exist, how long code can run, how much memory it has, and what files or secrets it can access. A script that works in Node may fail in the browser; a serverless function may fail if it assumes a long-lived process.
Plainly
Think of Runtime as roads and power for the app city. The environment where code actually executes: browser, Node.js, edge worker, serverless function, or mobile app.
In practice
Use it when local behavior needs to become a reachable, reliable deployed service. In practice, define the owner, input, output, and failure mode before you rely on it.