This vessel (GOCE) is in orbit and maneuvers by using air similar to a plane to create lift and turn. The solar powered ion thrusters powers it continuously inside the atmosphere for years. Is this technically a plane, satellite, or a missile?
A satellite is an object in orbit. A satellite keeps a constant altitude if it is traveling at the correct orbital speed for their altitude. Otherwise it will be in an elliptical orbit where the altitude is constantly changing. In any case, satellites are (almost) in free-fall
An airplane travels at much, much lower speed than orbital speed. Airplanes keep their altitude because their wings produce lift.
Satellites in lower orbited are subject to atmospheric drag, albeit to much smaller extentd than planes. GOCE was orbiting Earth at an - for satellites - very low altitude (224 km). There is hardly any atmosphere at this altitude, but at an orbital velocity of 7.8 m/s (51337 kts), atmospheric drag is still a problem.
Therefore the structure of GOCE was particularly designed for low drag. The wings were flat without camber. They did not produce any lift. GOCE used ion thrusters to compensate the atmospheric drag and maintain orbital velocity.
Missiles are self-propelled guided weapons. I can assure you that GOCE was not a weapon.
[TL;DR]: GOCE was a satellite, because it orbited an astronomical body. It was not a plane, because its wings did not produce lift.
GOCE´s 20 month mission ended on November 11, 2013 with the planned destructive re-enty into the atmosphere. It measured Earth´s gravity field and acquired a precise model of the Earth geoid: