1. A smooth, flat, relatively thin, rigid body of uniform thickness. 2. a. A sheet of hammered, rolled, or cast metal. b. A very thin applied or deposited coat of metal. 3. a. A flat piece of metal forming part of a machine: a boiler plate. b. A flat piece of metal on which something is engraved. c. A license plate: a car with Utah plates. 4. a.