Mission design is the process of designing a space mission to fulfill a particular objective.
Mission design (more broadly, mission analysis and design) is the (typically, iterative and gradual, yet systematic) process of finding an engineering solution to a complex scientific, technical, military, political, or social problem that potentially involves a spacecraft. It is meant to minimize the cost of a space mission or the "system of systems" where it belongs while meeting the objectives and constraints set by mission planners and customers.