Civilian

A civilian was any non-military, non-law enforcement or non-Starfleet personnel member who went about their everyday lives in the presence of the aforementioned.
Sometimes Starfleet would coordinate with civilian medical or scientific organizations to share lab space onboard Starfleet vessels. Crew that were civilians were considered among the lowest ranking and even a Crewman could order one about however in practice Starfleet personnel understood that civilians were filling specialized research roles and were mostly left alone. 
In extreme emergencies, civilians, especially those with medical training, could be drafted to form medical or engineering teams although this was often considered a measure of last resort.