Medical terminologies and vocabularies describing the semantics of medical knowledge aim to improve the communication of ideas between clinicians and researchers. Often more than one organization will produce terminologies that cover the same conceptual domain. This complicates the goal of communicating ideas with a common understanding of the intended meaning. In situations like these, it is important to define the interface between two overlapping medical vocabularies to avoid semantic drift when converting between terminologies. This paper describes a method for defining the interface between overlapping terminologies using XML.