A Mission Specialist (MS) is a position held by certain NASA astronauts during Space Shuttle missions. A Mission Specialist is assigned to a limited field of the mission, such as for medical experiments or technical quests.
Other functions on board are Pilot, Flight Engineer and Mission Commander. Some Space Shuttle missions included Payload Specialists in addition to Mission Specialists. While a Payload Specialist was selected for a single specific mission, a Mission Specialist was selected as an astronaut first, and then assigned to a mission.
Mission Specialist is abbreviated by NASA as MS.