According to Wikipedia:
Master data represents the business objects that contain the most valuable, agreed upon information shared across an organization. It gives context to business activities and transactions, answering questions like who, what, when and how as well as expanding the ability to make sense of these activities through categorizations, groupings and hierarchies. It can cover relatively static reference data, transactional, unstructured, analytical, hierarchical and metadata. It is the primary focus of the information technology (IT) discipline of master data management (MDM).
But since the creation of more and more (micro-)services, gray areas arise:
Master data is usually non-transactional in nature, but in some cases gray areas exist where transactional processes and operations may be considered master data by an organization. For example, master data may contain information about customers, products, employees, materials, suppliers, and vendors. Though rare, if that information is only contained within transactional data such as orders and receipts and is not housed separately, it may be considered master data.
Of course in the real world, things are more often gray. At Monkey Master Data we feel that even entities such as Consumers (identical to Customers or Users for example) are considered master data. Since many of the datasets originate not just from a single service or application, but actually are combined datasets as seen below in an example:
If you have any questions, feel free to contact us.