As used in this chapter, the following terms shall have the meanings indicated:
An artificially constructed barrier of any material or combination of materials erected to enclose, screen, or separate areas. A fence shall include all gates. Fences shall be considered as being incidental structures to a principal permitted use or to a principal permitted structure or principal permitted building and are permitted in all zones in accordance with the criteria set forth in this chapter.
An artificially constructed structure of any material or combination of materials constructed between lands of different elevations to effect a sudden grade change, stabilize the surfaces, prevent erosion and/or protect structures. A retaining wall creating a change of grade less than 48 inches at the maximum differential (top of wall to bottom of wall) is not considered to be a structure.