The Collaborative International Dictionary of English v.0.48:

Yawn \Yawn\, n.
   1. An involuntary act, excited by drowsiness, etc.,
      consisting of a deep and long inspiration following
      several successive attempts at inspiration, the mouth,
      fauces, etc., being wide open.
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            One person yawning in company will produce a
            spontaneous yawn in all present.      --N. Chipman.
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   2. The act of opening wide, or of gaping. --Addison.
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   3. A chasm, mouth, or passageway. [R.]
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            Now gape the graves, and trough their yawns let
            loose
            Imprisoned spirits.                   --Marston.
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The Collaborative International Dictionary of English v.0.48:

Yawn \Yawn\ (y[add]n), v. i. [imp. & p. p. Yawned; p. pr. &
   vb. n. Yawning.] [OE. yanien, [yogh]anien, ganien, gonien,
   AS. g[=a]nian; akin to ginian to yawn, g[imac]nan to yawn,
   open wide, G. g[aum]hnen to yawn, OHG. gin[=e]n, gein[=o]n,
   Icel. g[imac]na to yawn, gin the mouth, OSlav. zijati to
   yawn, L. hiare to gape, yawn; and perhaps to E. begin, cf.
   Gr. cheia` a hole. [root]47b. Cf. Begin, Gin to begin,
   Hiatus.]
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   1. To open the mouth involuntarily through drowsiness,
      dullness, or fatigue; to gape; to oscitate. "The lazy,
      yawning drone." --Shak.
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            And while above he spends his breath,
            The yawning audience nod beneath.     --Trumbull.
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   2. To open wide; to gape, as if to allow the entrance or exit
      of anything.
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            't is now the very witching time of night,
            When churchyards yawn.                --Shak.
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   3. To open the mouth, or to gape, through surprise or
      bewilderment. --Shak.
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   4. To be eager; to desire to swallow anything; to express
      desire by yawning; as, to yawn for fat livings. "One long,
      yawning gaze." --Landor.
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WordNet (r) 3.0 (2006):

yawn
    n 1: an involuntary intake of breath through a wide open mouth;
         usually triggered by fatigue or boredom; "he could not
         suppress a yawn"; "the yawning in the audience told him it
         was time to stop"; "he apologized for his oscitancy" [syn:
         yawn, yawning, oscitance, oscitancy]
    v 1: utter a yawn, as from lack of oxygen or when one is tired;
         "The child yawned during the long performance"
    2: be wide open; "the deep gaping canyon" [syn: gape, yawn,
       yaw]