A mission specialist was a position in Starfleet held by either Starfleet personnel or civilians during certain missions.
In 2366, Tam Elbrun transferred to the USS Enterprise-D as a mission specialist for a first contact mission. (TNG: "Tin Man")
In 2368 Ensign Robin Lefler served as mission specialist on board the Enterprise for its survey of the Phoenix Cluster. (TNG: "The Game")