The Collaborative International Dictionary of English v.0.48:

Legion \Le"gion\ (l[=e]"j[u^]n), n. [OE. legioun, OF. legion, F.
   l['e]gion, fr. L. legio, fr. legere to gather, collect. See
   Legend.]
   1. (Rom. Antiq.) A body of foot soldiers and cavalry
      consisting of different numbers at different periods, --
      from about four thousand to about six thousand men, -- the
      cavalry being about one tenth.
      [1913 Webster]

   2. A military force; an army; military bands.
      [1913 Webster]

   3. A great number; a multitude.
      [1913 Webster]

            Where one sin has entered, legions will force their
            way through the same breach.          --Rogers.
      [1913 Webster]

   4. (Taxonomy) A group of orders inferior to a class.
      [1913 Webster]

   Legion of honor, an order instituted by the French
      government in 1802, when Bonaparte was First Consul, as a
      reward for merit, both civil and military.
      [1913 Webster]

WordNet (r) 3.0 (2006):

legion
    adj 1: amounting to a large indefinite number; "numerous times";
           "the family was numerous"; "Palomar's fans are legion"
           [syn: numerous, legion(p)]
    n 1: archaic terms for army [syn: host, legion]
    2: association of ex-servicemen; "the American Legion"
    3: a large military unit; "the French Foreign Legion"
    4: a vast multitude [syn: horde, host, legion]

Moby Thesaurus II by Grady Ward, 1.0:

112 Moby Thesaurus words for "legion":
   KP, a mass of, a world of, armed force, armed service, army,
   army group, array, battalion, battery, battle group, bevy, brigade,
   bunch, cadre, career soldiers, cloud, cluster, clutter, cohort,
   cohue, column, combat command, combat team, company, corps, covey,
   crowd, crush, deluge, detachment, detail, division, field army,
   field train, fighting machine, file, flight, flock, flocks, flood,
   flying column, forces, galaxy, garrison, ground forces,
   ground troops, hail, heap, hive, horde, host, jam, kitchen police,
   large amount, legions, lots, maniple, many, mass, masses of,
   military establishment, mob, muchness, multitude, nest, numbers,
   occupation force, organization, outfit, pack, panoply, paratroops,
   phalanx, platoon, plurality, posse, press, quantities, quite a few,
   rabble, rank, rank and file, ranks, regiment, regular army,
   regulars, rout, ruck, scores, section, shoal, ski troops, soldiery,
   spate, squad, squadron, standing army, storm troops, swarm,
   tactical unit, task force, the line, the military, throng,
   tidy sum, train, troop, troops, unit, wing, worlds of



Easton's 1897 Bible Dictionary:

Legion
   a regiment of the Roman army, the number of men composing which
   differed at different times. It originally consisted of three
   thousand men, but in the time of Christ consisted of six
   thousand, exclusive of horsemen, who were in number a tenth of
   the foot-men. The word is used (Matt. 26:53; Mark 5:9) to
   express simply a great multitude.