Trajectory design is the process of designing the trajectory of a space mission so as to fulfill the larger design objectives for that mission.

As an example, consider OSIRIS-REx, whose key goal was to return a sample from some near Earth asteroid. Along with design of the trajectory, the mission design included the selection of the asteroid, the selection of sensors needed to accomplish the goal, the design of the collection mechanism, the electrical power capabilities and onboard computation capabilities needed to support the mission, and many other things. Mission design is a very broad topic while trajectory design is a much narrower topic.