The Collaborative International Dictionary of English v.0.48:

Troop \Troop\, n. [F. troupe, OF. trope, trupe, LL. troppus; of
   uncertain origin; cf. Icel. [thorn]orp a hamlet, village, G.
   dorf a village, dial. G. dorf a meeting. Norw. torp a little
   farm, a crowd, E. thorp. Cf. Troupe.]
   1. A collection of people; a company; a number; a multitude.
      [1913 Webster]

            That which should accompany old age
            As honor, love, obedience, troops of friends
            I must not look to have.              --Shak.
      [1913 Webster]

   2. Soldiers, collectively; an army; -- now generally used in
      the plural.
      [1913 Webster]

            Farewell the plumed troop, and the big wars. --Shak.
      [1913 Webster]

            His troops moved to victory with the precision of
            machines.                             --Macaulay.
      [1913 Webster]

   3. (Mil.) Specifically, a small body of cavalry, light horse,
      or dragoons, consisting usually of about sixty men,
      commanded by a captain; the unit of formation of cavalry,
      corresponding to the company in infantry. Formerly, also,
      a company of horse artillery; a battery.
      [1913 Webster]

   4. A company of stageplayers; a troupe. --W. Coxe.
      [1913 Webster]

   5. (Mil.) A particular roll of the drum; a quick march.
      [1913 Webster]

   6. See Boy scout, above.
      [Webster 1913 Suppl.]

The Collaborative International Dictionary of English v.0.48:

Troop \Troop\, v. i. [imp. & p. p. Trooped; p. pr. & vb. n.
   Trooping.]
   1. To move in numbers; to come or gather in crowds or troops.
      "Armies . . . troop to their standard." --Milton.
      [1913 Webster]

   2. To march on; to go forward in haste.
      [1913 Webster]

            Nor do I, as an enemy to peace,
            Troop in the throngs of military men. --Shak.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Troop \Troop\, v. t.

   To troop the colors or To troop the colours (Mil.),
   in the British army, to perform a ceremony consisting
   essentially in carrying the colors, accompanied by the band
   and escort, slowly before the troops drawn up in single file
   and usually in a hollow square, as in London on the
   sovereign's birthday.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

troop
    n 1: a group of soldiers
    2: a cavalry unit corresponding to an infantry company
    3: a unit of Girl or Boy Scouts [syn: troop, scout troop,
       scout group]
    4: an orderly crowd; "a troop of children" [syn: troop,
       flock]
    v 1: march in a procession; "the veterans paraded down the
         street" [syn: parade, troop, promenade]
    2: move or march as if in a crowd; "They children trooped into
       the room"