The Collaborative International Dictionary of English v.0.48:

Picket \Pick"et\, n. [F. piquet, properly dim. of pique spear,
   pike. See Pike, and cf. Piquet.]
   1. A stake sharpened or pointed, especially one used in
      fortification and encampments, to mark bounds and angles;
      or one used for tethering horses.
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   2. A pointed pale, used in marking fences.
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   3. [Probably so called from the picketing of the horses.]
      (Mil.) A detached body of troops serving to guard an army
      from surprise, and to oppose reconnoitering parties of the
      enemy; -- called also outlying picket.
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   4. By extension, men appointed by a trades union, or other
      labor organization, to intercept outsiders, and prevent
      them from working for employers with whom the organization
      is at variance. [Cant]
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   5. A military punishment, formerly resorted to, in which the
      offender was forced to stand with one foot on a pointed
      stake.
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   6. A game at cards. See Piquet.
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   Inlying picket (Mil.), a detachment of troops held in camp
      or quarters, detailed to march if called upon.

   Picket fence, a fence made of pickets. See def. 2, above.


   Picket guard (Mil.), a guard of horse and foot, always in
      readiness in case of alarm.

   Picket line. (Mil.)
      (a) A position held and guarded by small bodies of men
          placed at intervals.
      (b) A rope to which horses are secured when groomed.

   Picketpin, an iron pin for picketing horses.
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The Collaborative International Dictionary of English v.0.48:

Picket \Pick"et\, v. t. [imp. & p. p. Picketed; p. pr. & vb.
   n. Picketing.]
   1. To fortify with pointed stakes.
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   2. To inclose or fence with pickets or pales.
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   3. To tether to, or as to, a picket; as, to picket a horse.
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   4. To guard, as a camp or road, by an outlying picket.
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   5. To torture by compelling to stand with one foot on a
      pointed stake. [Obs.]
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The Collaborative International Dictionary of English v.0.48:

Piquet \Pi*quet"\, n. [F., prob. fr. pique. See Pique, Pike,
   and Picket.]
   A game at cards played between two persons, with thirty-two
   cards, all the deuces, threes, fours, fives, and sixes, being
   set aside. [Written also picket and picquet.]
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WordNet (r) 3.0 (2006):

picket
    n 1: a person employed to keep watch for some anticipated event
         [syn: lookout, lookout man, sentinel, sentry,
         watch, spotter, scout, picket]
    2: a detachment of troops guarding an army from surprise attack
    3: a protester posted by a labor organization outside a place of
       work
    4: a vehicle performing sentinel duty
    5: a wooden strip forming part of a fence [syn: picket,
       pale]
    6: a form of military punishment used by the British in the late
       17th century in which a soldier was forced to stand on one
       foot on a pointed stake [syn: picket, piquet]
    v 1: serve as pickets or post pickets; "picket a business to
         protest the layoffs"
    2: fasten with a picket; "picket the goat"