Any body travelling through particles undergoes drag. Any body able to generate lift (for instance spheres cannot generate lift) can generate lift if it undergoes drag.
First by assuming one body in a circular orbit around Earth at 250km altitude. Assuming this body has a non-zero lift to drag ratio, is able to control its attitude on three axis, and is able to provide constant thrust for multiple orbits.
If thrust is used to push this body in the same direction as its velocity vector, and its lift is steered to counter Earth's gravity, then it should be able to orbit slower than its orbital velocity, which means it's flying, rather than being in an ever missing Earth free fall / orbit.
Does this make it an aircraft? (deliberately ignoring Karman line/altitude definitions)
Secondly assuming the same body in a higher circular orbit in very thin exosphere: When does atmospheric drag and lift become less influent than drag and lift generated by solar radiation pressure? Is there an altitude at which this body cannot be an aircraft anymore, because dominant force acting on it is SRP, therefore it becomes a solarcraft?
Shouldn't this very region of high atmosphere become the boundary between flight and spaceflight?
Edit: This question is not about redefining Karman line or redefining what an aircraft, or a spacecraft should be characterized by.
This question is more about lift in its very essence, in the first place. Everything else are consequences.
I made the premise that every medium generating drag or pressure to some body moving into it, allows this body (if designed and oriented to do so) to generate lift. Is this correct?
Why can solar radiation pressure generate lift, but very thin atmosphere couldn't? why can particles travelling at light speed generate lift by hitting some slanted surface, when ~5-10km/s thin atmospheric particles produce drag and only drag?
I remember this comment to an answer to this question :
"Indeed it is true that "aerodynamics of gliding works about the same at ... 0.1 bar and 0.001 bar" but somewhere before you get to ~10E-6 bar that all changes: the gas becomes non-collisional, i.e the mean free path of the molecules or atoms is much larger than the scale of the vehicle. Then continuum aerodynamics doesn't work anymore. Notably, the mechanism for generating lift is very different, relying on momentum exchange from molecules or atoms impacting the surface of the vehicle. The usual mechanism of impact-adsorption-reemission at thermal velocities is very inefficient"
So why couldn't some spacecraft orbiting circular in leo, and having wings oriented with a positive angle of attack relative to Earth (like an airplane's wing relative to horizon) "fly" slower* than its intended orbital velocity, thanks to thrust provided by some engine, for instance air-breathing electric propulsion, or whatever kind of long duration, very low thrust engine?
*even if it flies at 7.123km/s without decaying, and should theoretically orbit unpowered at 7.124km/s
ADDENDUM: my interpretation of "lift" may be wrong, but it implies any kind of thrust generated by deflecting particles. For instance, a deeply stalled airfoil in dense atmosphere at 50deg angle of attack still produces some residual "lift" which should be defined by "thrust".