backplate

noun

1. A plate protecting a fighting person's back.

   “The cuirass of Roman muscled type is composed of an anatomically modeled breastplate and backplate, each made in one with a gorget […]”

2. A plate harnessed to a diver's back, to which the gas cylinders are attached.
3. The metal plate on the rear of a computer's casing, where the ports are found.
4. A wood or metal (or glass, etc) plate that serves as a backing for an object.
5. (UK)A rigid metal plate inside the brake drum, which supports the wheel cylinder, brake shoes, and other parts of a drum brake.
6. Part of a condenser microphone that sits behind the diaphragm and which acts, in conjunction with the diaphragm, as a sort of variable capacitor.