NOUN (3)

1. a military unit that is a subdivision of a company; usually has a headquarters and two or more squads; usually commanded by a lieutenant;

2. a team of policemen working under the military platoon system;

3. a group of persons who are engaged in a common activity;
- Example: "platoons of tourists poured out of the busses"
- Example: "the defensive platoon of the football team"

The Collaborative International Dictionary of English v.0.48:

Platoon \Pla*toon"\, n. [F. peloton a ball of thread, a knot or
   group of men, a platoon, from pelote a ball formed of things
   wound round. See Pellet.] (Mil.)
   (a) Formerly, a body of men who fired together; also, a small
       square body of soldiers to strengthen the angles of a
       hollow square.
   (b) Now, in the United States service, half of a company.
       [1913 Webster]

WordNet (r) 3.0 (2006):

platoon
    n 1: a military unit that is a subdivision of a company; usually
         has a headquarters and two or more squads; usually
         commanded by a lieutenant
    2: a team of policemen working under the military platoon system
    3: a group of persons who are engaged in a common activity;
       "platoons of tourists poured out of the busses"; "the
       defensive platoon of the football team"