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.
      [1913 Webster]

   2. A pointed pale, used in marking fences.
      [1913 Webster]

   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.
      [1913 Webster]

   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]
      [1913 Webster]

   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.
      [1913 Webster]

   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.
      [1913 Webster]

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.
      [1913 Webster]

   2. To inclose or fence with pickets or pales.
      [1913 Webster]

   3. To tether to, or as to, a picket; as, to picket a horse.
      [1913 Webster]

   4. To guard, as a camp or road, by an outlying picket.
      [1913 Webster]

   5. To torture by compelling to stand with one foot on a
      pointed stake. [Obs.]
      [1913 Webster]

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.]
   [1913 Webster]

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"

Moby Thesaurus II by Grady Ward, 1.0:

167 Moby Thesaurus words for "picket":
   Argus, Charley, advance guard, airplane spotter, anchor,
   armed guard, bank guard, bar, beef, bind, bitch, blockade, boggle,
   box in, boycott, bridle, bulkhead in, call in question, chain,
   challenge, coast guard, complain, cordon, cordon sanitaire,
   cry out against, demonstrate, demonstrate against, demonstrator,
   demur, dismember, dispute, draw and quarter, enchain, enclose,
   enter a protest, entrammel, expostulate, fasten, fence, fence in,
   fetter, fire patrolman, fire warden, fireguard, flying squadron,
   forward observer, garrison, go on strike, go out, goalie,
   goalkeeper, goaltender, goon, goon squad, grill, guard, guarder,
   guardsman, gyve, hamper, handcuff, hedge, hem, hem in, hobble,
   hog-tie, hold out, holler, hopple, howl, impale, inlying picket,
   jailer, keelhaul, kick, lash, leash, leg, lock out, lookout,
   lookout man, make fast, manacle, march, moor, night watchman,
   object, observer, outguard, outpost, pale, paling, palisade,
   patrol, patroller, patrolman, peg, peg down, picket line, picketer,
   picquet, pile, pin down, pinion, post, press objections, protest,
   protester, put in irons, rack, rail, raise a howl, rally,
   rear guard, remonstrate, restrain, revolt, rope, roundsman, scout,
   scruple, secure, security guard, sentinel, sentry, shackle, shank,
   shut in, shut it down, sit down, sit in, slow down, spile, spotter,
   squawk, stake, stanchion, state a grievance, straitjacket, strap,
   strike, striker, strong-arm man, tar and feather, teach in, tether,
   tie, tie down, tie up, torture, train guard, trammel, upright, van,
   vanguard, vedette, vertical, walk out, wall, wall in, ward, warder,
   watch, watcher, watchkeeper, watchman, yell bloody murder