) An optional feature of the HTTP protocol that speeds up requests by ensuring that the data stored in a Web cache are up-to-date. The ETag is an identifier assigned to a data resource in a server, and if that resource is ever updated at the server, the ETag is changed.
Whenever a resource is requested (via its URL), the data and ETag are retrieved and stored in the Web cache, and the ETag is sent along with subsequent requests. If the ETag at the server has not changed, a "Not Modified" message is returned, and the cached data are used. See Web cache
and browser cache