The Collaborative International Dictionary of English v.0.48:

Plate \Plate\, n. [OF. plate a plate of metal, a cuirsas, F.
   plat a plate, a shallow vessel of silver, other metal, or
   earth, fr. plat flat, Gr. ?. See Place, n.]
   1. A flat, or nearly flat, piece of metal, the thickness of
      which is small in comparison with the other dimensions; a
      thick sheet of metal; as, a steel plate.
      [1913 Webster]

   2. Metallic armor composed of broad pieces.
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            Mangled . . . through plate and mail. --Milton.
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   3. Domestic vessels and utensils, as flagons, dishes, cups,
      etc., wrought in gold or silver.
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   4. Metallic ware which is plated, in distinction from that
      which is silver or gold throughout.
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   5. A small, shallow, and usually circular, vessel of metal or
      wood, or of earth glazed and baked, from which food is
      eaten at table.
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   6. [Cf. Sp. plata silver.] A piece of money, usually silver
      money. [Obs.] "Realms and islands were as plates dropp'd
      from his pocket." --Shak.
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   7. A piece of metal on which anything is engraved for the
      purpose of being printed; hence, an impression from the
      engraved metal; as, a book illustrated with plates; a
      fashion plate.
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   8. A page of stereotype, electrotype, or the like, for
      printing from; as, publisher's plates.
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   9. That part of an artificial set of teeth which fits to the
      mouth, and holds the teeth in place. It may be of gold,
      platinum, silver, rubber, celluloid, etc.
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   10. (Arch.) A horizontal timber laid upon a wall, or upon
       corbels projecting from a wall, and supporting the ends
       of other timbers; also used specifically of the roof
       plate which supports the ends of the roof trusses or, in
       simple work, the feet of the rafters.
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   11. (Her.) A roundel of silver or tinctured argent.
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   12. (Photog.) A sheet of glass, porcelain, metal, etc., with
       a coating that is sensitive to light.
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   13. A prize giving to the winner in a contest.
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   14. (Baseball) A small five-sided area (enveloping a
       diamond-shaped area one foot square) beside which the
       batter stands and which must be touched by some part of a
       player on completing a run; -- called also home base,
       or home plate.
       [Webster 1913 Suppl.]

   15. One of the thin parts of the bricket of an animal.
       [Webster 1913 Suppl.]

   16. A very light steel racing horsehoe.
       [Webster 1913 Suppl.]

   17. Loosely, a sporting contest for a prize; specif., in
       horse racing, a race for a prize, the contestants not
       making a stake.
       [Webster 1913 Suppl.]

   18. Skins for fur linings of garments, sewed together and
       roughly shaped, but not finally cut or fitted. [Furrier's
       Cant]
       [Webster 1913 Suppl.]

   19. (Hat Making) The fine nap (as of beaver, hare's wool,
       musquash, nutria, or English black wool) on a hat the
       body of which is of an inferior substance.
       [Webster 1913 Suppl.]

   20. a quantity sufficient to fill a plate; a plateful; a
       dish containing that quantity; a plate of spaghetti.
       [PJC]

   21. the food and service supplied to a customer at a
       restaurant; as, the turkey dinner is $9 a plate; I'll
       have a plate of spaghetti.
       [PJC]

   22. a flat dish of glass or plastic with a fitted cover, used
       for culturing microorganisms in a laboratory.
       [PJC]

   23. the identification tag required to be displayed on the
       outside of a vehicle; same as license plate; -- often
       used in the plural.
       [PJC]

   24. an agenda or schedule of tasks to be performed; I have a
       lot on my plate today. [colloq.]
       [PJC]

   Note: Plate is sometimes used in an adjectival sense or in
         combination, the phrase or compound being in most cases
         of obvious signification; as, plate basket or
         plate-basket, plate rack or plate-rack.
         [1913 Webster]

   Home plate. (Baseball) See Home base, under Home.

   Plate armor.
       (a) See Plate, n., 2.
       (b) Strong metal plates for protecting war vessels,
           fortifications, and the like.

   Plate bone, the shoulder blade, or scapula.

   Plate girder, a girder, the web of which is formed of a
      single vertical plate, or of a series of such plates
      riveted together.

   Plate glass. See under Glass.

   Plate iron, wrought iron plates.

   Plate layer, a workman who lays down the rails of a railway
      and fixes them to the sleepers or ties.

   Plate mark, a special mark or emblematic figure stamped
      upon gold or silver plate, to indicate the place of
      manufacture, the degree of purity, and the like; thus, the
      local mark for London is a lion.

   Plate paper, a heavy spongy paper, for printing from
      engraved plates. --Fairholt.

   Plate press, a press with a flat carriage and a roller, --
      used for printing from engraved steel or copper plates.

   Plate printer, one who prints from engraved plates.

   Plate printing, the act or process of printing from an
      engraved plate or plates.

   Plate tracery. (Arch.) See under Tracery.

   Plate wheel (Mech.), a wheel, the rim and hub of which are
      connected by a continuous plate of metal, instead of by
      arms or spokes.
      [1913 Webster]

WordNet (r) 3.0 (2006):

plate armor
    n 1: specially hardened steel plate used to protect
         fortifications or vehicles from enemy fire [syn: armor
         plate, armour plate, armor plating, plate armor,
         plate armour]