In system-of-systems (SoS), a mission is an essential information that can guide the whole SoS development process. Through the so-called mission model, it is possible to identify required capabilities for the constituent system, operations, connections, emergent behavior, among other elements that characterize a SoS. Although the importance of missions for the SoS domain, the literature provides few proposals that focus on mission and none of them encompass a conceptual model for representing missions or a language to define mission models. In this paper, we present mKAOS, a language for SoS mission description that is based on a conceptual model for SoS missions. mKAOS organizes the mission-related information in a set of complementary models that allows a detailed description of mission information independent of implementation details. mKAOS also has an associated tool, mKAOS Studio, an open-source tool for modeling SoS missions using the mKAOS language.