This paper addresses the importance of a unified ontology for a Battle Command (BC) system of systems (SoS) acquisition. A BC SoS is a Command, Control, Communications, Computers, Intelligence, Surveillance, Reconnaissance and Target Acquisition (C4ISR&TA) federation of large-scale, net-centric systems that are collaborative and interoperable and include heterogeneous multi-agency managed intelligent agents, humans-in-the-loop, and unmanned autonomous systems. As systems become increasingly complex, modularity becomes the key to reuse, scalability, and an open architecture. In addition, these design features are key to a manageable and affordable transformation from current to future capabilities across acquisition maturity phases over several decades of fielding. A new large-scale SoS cannot be built in isolation. It needs to evolve internally and accommodate external pressures to integrate or be interoperable with current systems of record. The development of a unifying ontology that spans multiple domains in the SoS is shown to be crucial, if not pivotal, to the success of SoS engineering efforts which are inherently multi-disciplinary and collaborative.