Wide angle is in comparison to 'normal'. On a full frame camera normal is considered to be 50mm and wide angle is less than 35mm.
Using these numbers we can determine that the accepted ratio for wide angle is less than 0.7 times that of normal on a given format. On APS-C the normal focal length is 35mm, making wide angle start at 23mm.
Most SLR cameras have a mount to film plane distance of about 40mm. Any lens with a focal length less than this distance requires a reverse telephoto group to extend the distance between the lens and the focal plane. On a full frame sensor a normal lens can be much simpler than a wide angle lens since a normal lens (50mm) has a greater focal length than the mount to film plane distance (40mm). On APS-C the normal lens (35mm) has a distance less than the mount to film plane distance, so there is no fundamental differences between the construction of a normal and wide angle lens.
Source: Wide Angle Lens on Wikipedia