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]