Backplate n. : A piece, or plate which forms the back of anything, or which covers the back; armor for the back.

Example:
Where can i buy a psp faceplate and Backplate?

Taxonomy
backplate, backplate and wing, wing, harness, BCD, cylinder, plate, buoyancy, waist strap, shoulder, inflation, bungee, rebreather, slots, D-ring, stainless steel, Steel plate, twin cylinders.

Defination from the web:
backplate and wing (often abbreviated as BP&W or BP/W), is a type of buoyancy compensation device (BCD) worn by scuba divers. ... The backplate, a plate normally made from metal which sits against the diver's back and to which the diver's cylinder or cylinders are attached. ...