Source : Webster's Revised Unabridged Dictionary (1913)

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.

   2. A pointed pale, used in marking fences.

   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}.

   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]