The HTTP ETag (entity tag) response header is an identifier for a specific version of a resource.
It lets caches be more efficient and save bandwidth, as a web server does not need to resend a full response if the content has not changed.
Additionally, ETags help to prevent simultaneous updates of a resource from overwriting each other ("mid-air collisions")....modification timestamp, or just a revision number. For example, a wiki...hexadecimal hash of the documentation article content. Examples...