Abstract
In the process of implementing a protocol for the transport of science data, the Open Source Project for a Network Data Access Protocol (OPeNDAP) group has learned a considerable amount about the internal anatomy of what are commonly considered monolithic concepts. In order to communicate among our group, we have adopted a collection of deinitions and observations about data and the metadata that make them useful: differentiating between "semantic" and "syntactic" metadata, and deining categories such as "translational" and "use" metadata. We share the deinitions and categorizations here in the hope that others will ind them as useful as we do.