The Collaborative International Dictionary of English v.0.48:

Crest \Crest\ (kr[e^]st), n. [OF. creste, F. cr[^e]te, L.
   crista.]
   1. A tuft, or other excrescence or natural ornament, growing
      on an animal's head; the comb of a cock; the swelling on
      the head of a serpent; the lengthened feathers of the
      crown or nape of bird, etc. --Darwin.
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            [Attack] his rising crest, and drive the serpent
            back.                                 --C. Pitt.
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   2. The plume of feathers, or other decoration, worn on a
      helmet; the distinctive ornament of a helmet, indicating
      the rank of the wearer; hence, also, the helmet.
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            Stooping low his lofty crest.         --Sir W.
                                                  Scott.
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            And on his head there stood upright
            A crest, in token of a knight.        --Gower.
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   3. (Her.) A bearing worn, not upon the shield, but usually
      above it, or separately as an ornament for plate,
      liveries, and the like. It is a relic of the ancient
      cognizance. See Cognizance, 4.
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   4. The upper curve of a horse's neck.
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            Throwing the base thong from his bending crest.
                                                  --Shak.
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   5. The ridge or top of a wave.
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            Like wave with crest of sparkling foam. --Sir W.
                                                  Scott.
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   6. The summit of a hill or mountain ridge.
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   7. The helm or head, as typical of a high spirit; pride;
      courage.
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            Now the time is come
            That France must vail her lofty plumed crest.
                                                  --Shak.
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   8. (Arch.) The ornamental finishing which surmounts the ridge
      of a roof, canopy, etc.
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            The finials of gables and pinnacles are sometimes
            called crests.                        --Parker.
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   9. (Engin.) The top line of a slope or embankment.
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   Crest tile, a tile made to cover the ridge of a roof,
      fitting upon it like a saddle.

   Interior crest (Fort.), the highest line of the parapet.
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The Collaborative International Dictionary of English v.0.48:

Crest \Crest\, v. t. [imp. & p. p. Crested; p. pr. & vb. n.
   Cresting.]
   1. To furnish with, or surmount as, a crest; to serve as a
      crest for.
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            His legs bestrid the ocean, his reared arm
            Crested the world.                    --Shak.
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            Mid groves of clouds that crest the mountain's brow.
                                                  --Wordsworth.
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   2. To mark with lines or streaks, like, or regarded as like,
      waving plumes.
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            Like as the shining sky in summer's night, . . .
            Is crested with lines of fiery light. --Spenser.
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The Collaborative International Dictionary of English v.0.48:

Crest \Crest\ (kr[e^]st), v. i.
   To form a crest.
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WordNet (r) 3.0 (2006):

crest
    n 1: the top line of a hill, mountain, or wave
    2: the top or extreme point of something (usually a mountain or
       hill); "the view from the peak was magnificent"; "they
       clambered to the tip of Monadnock"; "the region is a few
       molecules wide at the summit" [syn: peak, crown, crest,
       top, tip, summit]
    3: the center of a cambered road [syn: crown, crest]
    4: (heraldry) in medieval times, an emblem used to decorate a
       helmet
    5: a showy growth of e.g. feathers or skin on the head of a bird
       or other animal
    v 1: lie at the top of; "Snow capped the mountains" [syn: cap,
         crest]
    2: reach a high point; "The river crested last night"