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.
      [1913 Webster]

            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.
      [1913 Webster]

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.]
   [1913 Webster]
   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.
      [1913 Webster]

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]

Moby Thesaurus II by Grady Ward, 1.0:

56 Moby Thesaurus words for "yawn":
   aperture, boredom, broaching, cavity, chasm, check, clearing,
   cleft, crack, dehisce, dehiscence, disclosure, doldrums, doze,
   drowse, ennui, fenestra, fistula, fontanel, foramen, gap, gape,
   gaping, gat, gulf, hang open, hiatus, hole, hollow, inlet,
   interval, lacuna, laying open, leak, nap, opening, opening up,
   orifice, oscitancy, oscitate, oscitation, outlet, pandiculation,
   passageway, pore, slot, snooze, space, split, stoma, the gapes,
   throwing open, uncorking, unstopping, yaw, yawning