Battleship is actually a contraction of the term Line of Battle Ship, or what was also known as a Ship of the Line. This term arises from naval tactics in the age of sail and cannon. Generally, when fleets fought each other, the only way you could direct your armament at the enemy was to present your side to their ships. They, of course, would do the same if they had the chance. Then all vessels present would try to get into a line and fire all cannons on the appropriate side at once in order to do the most damage to the enemy (this is where the term broadside comes from; it typically means a volley of fire from the guns on one side of a warship).

In any case, only the largest and strongest ships could survive this sort of ruthless pounding with any chance of outlasting the enemy. Thus, these ships were called ships of the line of battle. This was later contracted to battleship. The tactics of the line of battle and 'crossing the T' (or having your line's broadside fire down the front of an oncoming opponent, which was ideal positioning) survived until the advent of naval aviation and the surface-to-surface missile.