The Collaborative International Dictionary of English v.0.48:

Shade \Shade\, v. t. [imp. & p. p. Shaded; p. pr. & vb. n.
   Shading.]
   1. To shelter or screen by intercepting the rays of light; to
      keep off illumination from. --Milton.
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            I went to crop the sylvan scenes,
            And shade our altars with their leafy greens.
                                                  --Dryden.
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   2. To shelter; to cover from injury; to protect; to screen;
      to hide; as, to shade one's eyes.
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            Ere in our own house I do shade my head. --Shak.
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   3. To obscure; to dim the brightness of.
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            Thou shad'st
            The full blaze of thy beams.          --Milton.
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   4. To pain in obscure colors; to darken.
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   5. To mark with gradations of light or color.
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   6. To present a shadow or image of; to shadow forth; to
      represent. [Obs.]
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            [The goddess] in her person cunningly did shade
            That part of Justice which is Equity. --Spenser.
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The Collaborative International Dictionary of English v.0.48:

Shading \Shad"ing\, n.
   1. Act or process of making a shade.
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   2. That filling up which represents the effect of more or
      less darkness, expressing rotundity, projection, etc., in
      a picture or a drawing.
      [1913 Webster]

WordNet (r) 3.0 (2006):

shading
    n 1: graded markings that indicate light or shaded areas in a
         drawing or painting
    2: a gradation involving small or imperceptible differences
       between grades [syn: shading, blending]

Moby Thesaurus II by Grady Ward, 1.0:

82 Moby Thesaurus words for "shading":
   arrangement, atmosphere, balance, black spot, blackening,
   blocking the light, bloom, blooping, brushwork, clouding, color,
   composition, covering, curtaining, darkening, definition,
   denigration, design, dimming, draftsmanship, eclipsing,
   extinguishment, flare, fringe area, ghost, gradation, grading,
   graduation, granulation, grid, grouping, hard shadow, image,
   interlaced scanning, line, melanization, mixing, monitoring,
   multiple image, nigrification, noise, obfuscation, obnubilation,
   obscuration, obscurement, obumbration, occulting, overcast,
   overclouding, overshading, overshadowing, overshadowment,
   painterliness, perspective, picture, picture noise, picture shifts,
   rain, rolling, scanning, scanning pattern, scintillation,
   screening, shadow, shadowing, smirch, smirching, smudge, smudging,
   smut, smutch, smutching, snow, snowstorm, switching, technique,
   telecasting, televising, tone, treatment, values, veiling