A battalion is a unit of an army. One battalion usually consists of three or more companies and a headquarters.
The word battalion sounds a lot like battle, and that's a clue to its meaning: battalions are groups engaged in a battle. Specifically, a battalion is a smaller segment of an army. Within a battalion, there will usually be a headquarters, as well as a few smaller units, which are called companies. The members of a battalion will work closely together, whether they are attacking an enemy or defending a base. Veterans always remember the other soldiers in their battalion.