What is the meaning of Backplate?

A plate protecting a fighting person's back.

A plate harnessed to a diver's back, to which the gas cylinders are attached.

The metal plate on the rear of a computer's casing, where the ports are found.

A wood or metal (or glass, etc) plate that serves as a backing for an object.

A rigid metal plate inside the brake drum, which supports the wheel cylinder, brake shoes, and other parts of a drum brake.

Part of a condenser microphone that sits behind the diaphragm and which acts, in conjunction with the diaphragm, as a sort of variable capacitor.

Source: wiktionary.org