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The Collaborative International Dictionary
Surface plate

Surface \Sur"face`\, n. [F. See Sur-, and Face, and cf. Superficial.]

  1. The exterior part of anything that has length and breadth; one of the limits that bound a solid, esp. the upper face; superficies; the outside; as, the surface of the earth; the surface of a diamond; the surface of the body.

    The bright surface of this ethereous mold.
    --Milton.

  2. Hence, outward or external appearance.

    Vain and weak understandings, which penetrate no deeper than the surface.
    --V. Knox.

  3. (Geom.) A magnitude that has length and breadth without thickness; superficies; as, a plane surface; a spherical surface.

  4. (Fort.) That part of the side which is terminated by the flank prolonged, and the angle of the nearest bastion.
    --Stocqueler.

    Caustic surface, Heating surface, etc. See under Caustic, Heating, etc.

    Surface condensation, Surface condenser. See under Condensation, and Condenser.

    Surface gauge (Mach.), an instrument consisting of a standard having a flat base and carrying an adjustable pointer, for gauging the evenness of a surface or its height, or for marking a line parallel with a surface.

    Surface grub (Zo["o]l.), the larva of the great yellow underwing moth ( Triph[oe]na pronuba). It is often destructive to the roots of grasses and other plants.

    Surface plate (Mach.), a plate having an accurately dressed flat surface, used as a standard of flatness by which to test other surfaces.

    Surface printing, printing from a surface in relief, as from type, in distinction from plate printing, in which the ink is contained in engraved lines.

Wikipedia
Surface plate

A surface plate is a solid, flat plate used as the main horizontal reference plane for precision inspection, marking out (layout), and tooling setup. The surface plate is often used as the baseline for all measurements to the workpiece, therefore one primary surface is finished extremely flat with accuracy up to for a grade AA or AAA plate. Surface plates are a very common tool in the manufacturing industry and are often permanently attached to robotic type inspection devices such as a coordinate-measuring machine. Plates are typically square or rectangular. One current British Standard includes specifications for plates from 160 mm x 100 mm to 2500 mm x 1600 mm.