The term registry is widely used to refer to any database storing clinical information collected as a byproduct of patient care. Despite the use of this single characterizing term (registry), these databases exist in various forms and support functions ranging from biomedical informatics and clinical research, to public health, epidemiology and evidence-based clinical practice. This ambiguous terminology impacts the ability to locate and learn about specific types of registries; the goal of this research project was to develop a more useful categorization scheme for registries. We thoroughly analyzed peer-reviewed publications related to registries. From our findings, we created a detailed definition for registries in healthcare (medical data registries, or MDRs) and outlined a set of characteristics common to all such MDRs. This framework, MDR-OK, comprises five distinguishing features of registries, and is intended to provide a clear understanding of MDRs as a functional variety of databases, as well as to provide a framework for evaluation and categorization of registries and other data systems.