Presently the FireCloud dictionary contains a definition for "Workspace attribute," but does not contain a general definition of attribute (e.g. as it may be applied to other entities). There is mention of Entity attributes in the FireCloud Basics section but it's informal and descriptive, not precise.
Having a formal definition somewhere in the docs would be helpful, especially in the dictionary, and perhaps it could allude to the nature of attributes as metadata. A related suggestion is given here, about members in container entities.