representation
information intended to reflect a resource's state, in a format that can be communicated in a message — bytes plus metadata
You never GET a resource; you get a representation of it — a specific serialization of its current (or desired, or previous) state, described by representation metadata (Content-Type, Content-Language, Content-Encoding, validators like ETag). One resource can have many representations (HTML vs JSON, English vs French, gzipped vs not); content negotiation is how client and server pick one.