An orbital engineer is an engineer who specializes in the field of orbital engineering; that is, the application of engineering science and technology to objects used or associated with a planet's orbit.
While posing as a crewman aboard Enterprise, Daniels claimed that he had a brother who was an orbital engineer at Jupiter Station. When it was later revealed that he was actually a time traveler from the future, he told Charles Tucker that he did not actually have a brother. (ENT: "Cold Front")
The ability to maintain operation of orbital tethers is within the confines of the skills possessed by an orbital engineer. When Lillias was having trouble attempting to get a power relay of a mag-lev carriage on-line, in 2372, she said "I'm not an orbital engineer. I can't do this." Fortunately Neelix, who had some experience with such technology, was able to help her by recalibrating the pressure valve. (VOY: "Rise")