picket line
  警戒線,糾察線
  Pick·et n.
  1. A stake sharpened or pointed, especially one used in fortification and encampments, to mark bounds and angles; or one used for tethering horses.
  2. A pointed pale, used in marking fences.
  3.  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.
  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]
  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.
  picket line
       n : a line of people acting as pickets