Traumatic Brain Injury (TBI) rehabilitation interventions are very heterogeneous due to injury characteristics and pathology, patient demographics, healthcare settings, caregiver variability, and individualized, multi-discipline treatment plans. Consequently, comparing and generalizing the effectiveness of interventions is limited largely due to non-interoperable domain data. Addressing domain data interoperability through standardization can help unpack the "black-box" of rehabilitation treatment research. This paper describes the development of a foundational non-surgical, non-pharmaceutical ontology for TBI rehabilitation to facilitate domain interoperability. A conceptualization of the clinical domain was developed through a triangulation of data sources in order to create context and to serve as the underlying source for an ontology. A set of classes with primitive ontological relations based on the conceptualization was assembled in the Protégé Ontology Editor. The ontology is designed to facilitate further granularity of classes, properties, and instances as a collaborative hub for domain engagement. It is proposed that the ontology will aid in identification of effective rehabilitation components through the facilitation of comparative effectiveness research (CER).