Definitions

• Unpaved Surface: A surface, intended for aircraft operations, composed of unbound or natural materials. Unpaved surfaces may include gravel, coral, sand, clay, hard packed soil mixtures, grass, turf or sod. (Note: Unpaved surfaces have also been referred to within the aviation industry as Unimproved Runways.)
• Unpaved Runway: A runway pavement constructed with an unpaved surface.
• Unprepared Surface: Any naturally occurring surface used as a runway that has not been altered by man.

Source: Transport Canada