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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WordNet (r) 3.0 (2006):

picket line
    n 1: a line of people acting as pickets