In order to improve the interoperability and reusability of future defense models and simulations, the Conceptual Models of the Mission Space-Korea (CMMS-K) is being developed. CMMS-K is an ontology-based conceptual modeling framework for military mission spaces. This paper proposes a semantic language for describing a mission space model (MSM) as a main component of CMMS-K. A MSM is a conceptual model for military business processes and scenarios. Compared to previous approaches, the proposed MSM description language is designed to maximize the interoperability and reusability of MSMs. Specifically, it provides a scheme to represent military scenarios meaningfully with domain ontologies, which consist of entity and task ontologies. A MSM may be composed of an atomic process (action) and/or a composite process. The proposed method identifies and reuses military actions and tasks, which play a critical role in military scenarios, through the proposed task ontology. A task is defined as a composition of actions conducted by the same subject. To show the expressivity of the proposed MSM description language, a case study on a close air support scenario is illustrated.