Supination, opposite of pronation, is the position of a body part or the entire body being ventral side up. In the case of the foot, the plantar surface is the ventral surface.
The body is in a supine position when it is ventral side up. This would be laying down on the back, stomach up.
Supination in the foot is a triplanar position, caused by the motions of inversion, adduction, and plantarflexion. This could lead to a bow-legged stance. 
The hand is in a supine position when it is palm up.