Source : Webster's Revised Unabridged Dictionary (1913)

   5. A military punishment, formerly resorted to, in which the
      offender was forced to stand with one foot on a pointed
      stake.

   6. A game at cards. See {Piquet}.

   {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.

Source : WordNet®

picket line
     n : a line of people acting as pickets