ETag
termweb dev
An opaque identifier for a specific version of a resource, used to ask whether a cached copy is still current.
It turns a full transfer into a short question: the client sends the tag it holds, and the server answers not-modified if nothing changed. That is the cheapest bandwidth saving on the web. It has a second life in write operations, where sending the tag with an update means the change applies only if nobody else changed it first, which is optimistic concurrency implemented in a header.
Also known as: entity tag