User agent
Also called: UA string
The text a browser sends to identify itself, its version and operating system.
Definition
Websites sometimes change behavior based on the user agent. DevTools can fake it to imitate other devices, but that doesn't change the real rendering engine, so it can't replace testing on real browsers.
In practice
The site shows "Your browser is not supported" to the latest Firefox because it only checks for the word "Chrome" in the user agent.
Learn it in context
Lessons that use this term.