A maintenance engineer was someone who specialized in repairing damaged systems.
In 2267, maintenance engineers at the pergium production station on Janus VI were sent to repair machinery that the Horta had damaged. They were later found dead. (TOS: "The Devil in the Dark")
In 2373, Kira Nerys speculated that a missing Bajoran maintenance engineer might be in reality a changeling infiltrator. (DS9: "By Inferno's Light")