HTTP 304 – Not Modified
The resource has not changed since the version specified by the request headers, so no body is sent.
Common use case
A core caching mechanism: browsers send If-None-Match or If-Modified-Since headers, and the server replies with 304 to say "your cached copy is still good", saving bandwidth.
Example
HTTP/1.1 304 Not Modified
ETag: "33a64df"
History
Part of HTTP/1.0 (RFC 1945, 1996), refined with ETag-based validation in HTTP/1.1.
Did you know?
A 304 response must never include a body - it exists purely to say "use what you already have".