The Collaborative International Dictionary of English v.0.48:

Battalion \Bat*tal"ion\ (b[a^]t*t[a^]l"y[u^]n; 106), n. [F.
   bataillon, fr. It. battaglione. See Battalia.]
   1. A body of troops; esp. a body of troops or an army in
      battle array. [archaic] "The whole battalion views."
      --Milton.
      [1913 Webster +PJC]

   2. (Mil.) An infantry command of two or more companies, which
      is the tactical unit of the infantry, or the smallest
      command which is self-supporting upon the battlefield, and
      also the unit in which the strength of the infantry of an
      army is expressed.
      [1913 Webster]

   Note: In the United States army, since April 29, 1898, a
         battalion consists of four companies, and three
         battalions form a regiment. The term is also applied to
         two or more batteries of artillery combined into a
         single command.
         [Webster 1913 Suppl.]

The Collaborative International Dictionary of English v.0.48:

Battalion \Bat*tal"ion\, v. t.
   To form into battalions. [R.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

battalion
    n 1: an army unit usually consisting of a headquarters and three
         or more companies
    2: a large indefinite number; "a battalion of ants"; "a
       multitude of TV antennas"; "a plurality of religions" [syn:
       battalion, large number, multitude, plurality,
       pack]

Moby Thesaurus II by Grady Ward, 1.0:

75 Moby Thesaurus words for "battalion":
   KP, age group, army, army group, band, battery, battle group, bevy,
   body, brigade, bunch, cabal, cadre, cast, clique, cohort, column,
   combat command, combat team, company, complement, contingent,
   corps, coterie, covey, crew, crowd, detachment, detail, division,
   faction, field army, field train, file, fleet, flying column, gang,
   garrison, group, grouping, groupment, in-group, junta,
   kitchen police, legion, maniple, mob, movement, organization,
   out-group, outfit, pack, party, peer group, phalanx, platoon,
   posse, rank, regiment, salon, section, set, squad, squadron,
   stable, string, tactical unit, task force, team, train, tribe,
   troop, troupe, unit, wing