user agent
termweb dev
The client identifying itself in a header - a claim, not a fact, and one of the oldest lies on the web.
The string is famously accreted: browsers claim to be other browsers because sites once tested for names instead of capabilities, so almost every modern agent asserts a lineage it does not have. Two practical points survive that history. Anything security-relevant that depends on it is trivially bypassed, since the client chooses what to send. And it is a fingerprinting input, which is why browsers are now reducing what it reveals rather than adding to it.
Also known as: ua string