The Collaborative International Dictionary of English v.0.48:

Scale \Scale\, n. [Cf. AS. scealu, scalu, a shell, parings; akin
   to D. schaal, G. schale, OHG. scala, Dan. & Sw. skal a shell,
   Dan. skiael a fish scale, Goth. skalja tile, and E. shale,
   shell, and perhaps also to scale of a balance; but perhaps
   rather fr. OF. escale, escaile, F. ['e]caille scale of a
   fish, and ['e]cale shell of beans, pease, eggs, nuts, of
   German origin, and akin to Goth. skalja, G. schale. See
   Shale.]
   1. (Anat.) One of the small, thin, membranous, bony or horny
      pieces which form the covering of many fishes and
      reptiles, and some mammals, belonging to the dermal part
      of the skeleton, or dermoskeleton. See Cycloid,
      Ctenoid, and Ganoid.
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            Fish that, with their fins and shining scales,
            Glide under the green wave.           --Milton.
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   2. Hence, any layer or leaf of metal or other material,
      resembling in size and thinness the scale of a fish; as, a
      scale of iron, of bone, etc.
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   3. (Zool.) One of the small scalelike structures covering
      parts of some invertebrates, as those on the wings of
      Lepidoptera and on the body of Thysanura; the elytra of
      certain annelids. See Lepidoptera.
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   4. (Zool.) A scale insect. (See below.)
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   5. (Bot.) A small appendage like a rudimentary leaf,
      resembling the scales of a fish in form, and often in
      arrangement; as, the scale of a bud, of a pine cone, and
      the like. The name is also given to the chaff on the stems
      of ferns.
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   6. The thin metallic side plate of the handle of a
      pocketknife. See Illust. of Pocketknife.
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   7. An incrustation deposit on the inside of a vessel in which
      water is heated, as a steam boiler.
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   8. (Metal.) The thin oxide which forms on the surface of iron
      forgings. It consists essentially of the magnetic oxide,
      Fe3O4. Also, a similar coating upon other metals.
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   Covering scale (Zool.), a hydrophyllium.

   Ganoid scale. (Zool.) See under Ganoid.

   Scale armor (Mil.), armor made of small metallic scales
      overlapping, and fastened upon leather or cloth.

   Scale beetle (Zool.), the tiger beetle.

   Scale carp (Zool.), a carp having normal scales.

   Scale insect (Zool.), any one of numerous species of small
      hemipterous insects belonging to the family Coccidae, in
      which the females, when adult, become more or less
      scalelike in form. They are found upon the leaves and
      twigs of various trees and shrubs, and often do great
      damage to fruit trees. See Orange scale,under Orange.


   Scale moss (Bot.), any leafy-stemmed moss of the order
      Hepaticae; -- so called from the small imbricated
      scalelike leaves of most of the species. See Hepatica,
      2, and Jungermannia.
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WordNet (r) 3.0 (2006):

scale insect
    n 1: small homopterous insect that usually lives and feeds on
         plants and secretes a protective waxy covering