Runtime
The program that actually executes code, such as Node.js running JavaScript on a server, or a browser running it on a laptop.
What it tells you about a developer
Runtime names on a profile locate the work: Node.js means server-side JavaScript, and browser-only work means frontend. Developers who work across runtimes in one language are usually full-stack.
The same language can run in different runtimes. JavaScript runs in browsers, in Node.js on servers, and in newer runtimes like Bun and Deno. When a resume says Node.js, the language is JavaScript or TypeScript and the work is backend.