"Professional" camera appears to have a wide variety of definitions, but the general connotation is a camera with features and build quality that are commensurate with what one generally supposes the average professional photographer's needs, and that tends to first revolve around ruggedness/durability, since the average pro is generally presumed to be harder on his/her equipment than the average non-pro. Certainly I've seen it to be generally true that professional photographers look at their cameras as a tool by which they ply their trade. Non-pros have a tendency to wring their hands over every little scratch or acquired cosmetic defect (broadly generalizing here).
We can say that it's "any camera that a professional uses", but that doesn't reflect the reality of the various market segments.
We can say that it's "any camera that a professional uses", but that doesn't reflect the reality of the various market segments.