ETag (or entity tag) HTTP 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 was not changed. Additionally, etags help to prevent simultaneous updates of a resource from overwriting each other ("mid-air collisions").
If the resource at a given URL changes, a new
Etag value must be generated. A comparison of them can determine whether two representations of a resource are the same. Etags are therefore similar to fingerprints, and might also be used for tracking purposes by some servers. They might also be set to persist indefinitely by a tracking server.