Master Data is not Metadata. Metadata is data about data. It describes data but it isn’t generally considered business data. For example, customer metadata would describe the attributes of a customer entity, the datatype and size of the attributes, which programs produce the data, which programs consume the data, what business rules are enforced on the attributes, etc. In a BI environment, derivation, transformations, source system, and last load time are also important. The key thing to understand is that you can know the complete metadata for customers without knowing who a single customer is. Master Data, on the other hand, is the real business data. Customer master data is an authoritative list of customers.
