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.

Before the introduction of aircraft carriers, battleships were the largest naval surface combatants. Modern battleships evolved from the dreadnought-type ship of the line after World War I.

A modern battleship is characterised by its large size, its full armour, and its armament. To be respected as a serious battleship rather than a mere heavy cruiser, a ship needs at least six 305 mm (12'') guns mounted in turrets. Usually, the guns were far heavier, though. The Bismarck had 380 mm guns, the Iowa class had 406 mm guns, and the main batteries of the Japanese failures mentioned below were a whopping 465 mm. Then, a decent battleship needs medium artillery (in the 127 mm range), and an enormous lot of tiny anti-aircraft guns. Usually, battleships had one or two small spotter seaplanes, too.

A ship with the same armament as a battleship but with less armour and thus a higher maximum speed is called a battlecruiser.

The first modern battleship --at least that's what my trusty Brockhaus encyclopedia tells me-- was the HMS Hood, later sunk by the Bismarck. Discussions about whether the Hood was a battlecruiser rather than a battleship keep turning up in military newsgroups, and in this node, too, apparently. The biggest battleships ever built, the Japanese Yamato and Musashi were huge failures.

Today, battleships are dead. None are active anymore, and there is no purpose left to them. They were used sporadically for shore bombardment, but even for that better solutions are now available. No matter how many people keep claiming it, there are no plans by the US Navy to reactivate any of the old Missouri class battleships. In fact, it would be impossible by now.

Corrections to the more egregious errors in Mawa's writeup above:

1. "Modern battleships" is something of a contradiction in terms.
2. The basic pattern of a steam turbine-powered all-big-gun capital ship was set by HMS Dreadnought in 1906 and was refined until the last few were completed shortly after World War II. Many of the battleships which fought in the second World War in the British, US, Japanese and French navies were designed and built during World War I; some of the opening shots of the war were fired by a German pre-dreadnought battleship, the Schleswig-Holstein against Polish fortifications near Danzig.
3. The term "battleship" was current from the end of the ironclad era in the late 19th century, although it had been used occasionally a hundred years earlier. The OED offers a range of quotations featuring it from the first decade of the 20th century. (I think that mawa may be correct as regards the usage of the German equivalents Linienschiff and Schlachtschiff, though)
4. HMS Hood (built 1918) was built as a battlecruiser, not a battleship. She was not particularly innovative, merely slightly bigger than her predecessors.
5. The failure of the Yamato and Musashi can hardly be disassociated with the general collapse of the Imperial Japanese Navy in the face of overwhelming odds. It is nonetheless true that by that time the battleship in general had become by and large an expensive liability vulnerable to air and submarine attack.
6. Not quite written off yet: two or three battleships of the USS New Jersey class were dragged out of mothballs and used in a shore bombardment role during the 1991 Gulf War, inter alia as launch platforms for cruise missiles. Then again, it may just be showing my age that I regard 1991 as being more or less the present day.

Battleship is also a Hasbro board game, based off of Battleships. It is played by two people.

Set up

Each player has a 10x10 grid in which they place their ships. Each player has five ships that can be placed either horizontal or vertical. The ships are different sizes:

• One two unit ship
• Two three unit ships
• One four unit ship
• One five unit ship

Ships can't overlap, and each player does not reveal their ships placement to the opponent.

Game Play

Once all ships are placed, each player takes turns announcing a grid location. The other player says either "Hit" or "Miss"; meaning there is a part of a ship at that location. The orientation of the ship is not revealed. Once a ship has been hit in each of it's units, it is sunk. Once one player sinks all of the opponent's ships the game ends.

I teach math; 6th grade up through Geometry. I try to make it fun. The following rules are for versions of Hasbro's game Battleship that I use to teach graphing skills. The kids like them, and if anyone out there wants to use them (or further adapt them for their own devious purposes), go right ahead. Enjoy.

BATTLESHIP

For students learning to plot points on the coordinate plane

Each student needs a sheet of graph paper, pencil, and straightedge. Everyone should mark off a coordinate grid that runs from –10 to +10 on both the x and y axes (it is helpful to either demonstrate this on the blackboard/overhead projector, or assist each child individually. I guess you could provide them with a ready-made grid, but where would be the learning in that?) Within that 20x20 grid, each student then outlines four 3x3 squares—their ships. A ship is sunk when “hit” by a single shot. For instance, if the vertices of the ship were (0,0), (3,0), (3,3), and (0,3), either a point in the interior of that region (say, (2,2)) or one along its boundaries (1,3) would sink the ship.

Students take turns calling shots ( X, Y ) and the teacher records each shot and plots it on a master grid (on the blackboard or overhead). It is impossible to sink your own ship, and therefore strategically advantageous to call shots where your own ships are located—that way, the master grid drawn by the teacher (or student leader) shows shots fired in that area, and opponents are less likely to find your ships.

The winner of the game is the last person left with ships unsunk. (It is possible, but not necessary, to keep track of how many ships each individual manages to “shoot down”, and award points accordingly.) Even after a student’s ships are all sunk, sie may continue to play, firing shots from hir (fictional) aircraft.

BATTLESHIP--Advanced Version

For students who have learned to graph linear equations in the form y = mx + b; provides practice identifying slope and y-intercept.

Played as above; this time, set up the grids extending from –20 to +20 on both axes, but keeping the ships in the –10 to +10 range. Students fire shots in the form of lines by giving the slope (m) and y-intercept (b). Any ship touched by a line is sunk. For instance, a ship bounded by (0,0), (3,0), (3,3), and (0,3) could be sunk by the lines     y = x,     y = -1x,     y = -2x + 3,  etc. This game works best when students have not yet learned to graph horizontal and/or vertical lines—if they have, I usually forbid these shots, or else they’ll just systematically cover the grid with horizontal or vertical lines (which might be fun once, but gets old quickly, and they're not learning about slope.)

If you do end up using one of these games, let me know how it goes. Have fun.