A Mission Specialist is a position held by certain NASA

NASA

The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...

 astronaut

Astronaut

An astronaut or cosmonaut is a person trained by a human spaceflight program to command, pilot, or serve as a crew member of a spacecraft....

s during Space Shuttle

Space Shuttle

The Space Shuttle was a manned orbital rocket and spacecraft system operated by NASA on 135 missions from 1981 to 2011. The system combined rocket launch, orbital spacecraft, and re-entry spaceplane with modular add-ons...

 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

Aviator

An aviator is a person who flies an aircraft. The first recorded use of the term was in 1887, as a variation of 'aviation', from the Latin avis , coined in 1863 by G. de la Landelle in Aviation Ou Navigation Aérienne...

, Flight Engineer

Flight engineer

Flight engineers work in three types of aircraft: fixed-wing , rotary wing , and space flight .As airplanes became even larger requiring more engines and complex systems to operate, the workload on the two pilots became excessive during certain critical parts of the flight regime, notably takeoffs...

 and Mission Commander

Commander

Commander is a naval rank which is also sometimes used as a military title depending on the individual customs of a given military service. Commander is also used as a rank or title in some organizations outside of the armed forces, particularly in police and law enforcement.-Commander as a naval...

. Some Space Shuttle missions included Payload Specialist

Payload Specialist

A Payload Specialist ' was an individual selected and trained by commercial or research organizations for flights of a specific payload on a NASA Space Shuttle mission...

s 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.