A tank is a tracked armoured combat vehicle designed to engage enemies head-on, using direct fire from a large-caliber gun and supporting fire from machine guns. Heavy armour as well as a high degree of mobility give it survivability, while the tracks allow it to cross even rough terrain at high speeds.
Tanks were invented, and first used in World War I, by the British and French Armies to break the deadlock of the trenches; in fact, examples can be found throughout history of very similar armored combat vehicles, gradually evolving to assume the former role of cavalry on the battlefield: to either flank opposing positions with fast movement, or to penetrate defenses by massive concentration, enabling troops to cross the "swept zone" (area under hostile fire) more rapidly than infantry. Either movement may then be followed up by deep penetration into enemy rear areas, again supported by their high mobility. Tanks seldom operate alone, being organised into armoured units, usually in combined arms forces. Without such support, tanks are vulnerable to special anti-tank artillery, other tanks, anti-tank mines, and (at short ranges) infantry, as well as specialised anti-tank helicopters or close air support aircraft.
While tanks are expensive to operate and support, they remain among the most formidable and versatile weapons of the modern battlefield, both for their ability to engage other ground targets (including fortifications) and their shock value against infantry. Tanks and armour tactics have undergone many generations of evolution over nearly a century. Although weapons systems and armour continue to be developed, often at very high cost, many nations have reconsidered the need for such heavy weaponry in a period characterised by unconventional warfare.
There are at least three possible explanations of the origin of the name "tank". One is it first arose in British factories making the hulls of the first battle tanks: the workmen were given the impression they were constructing tracked water containers or tanks for the British Army, hence keeping the production of a fighting vehicle secret. Another is the term was first used in a secret report on the new motorized weapon presented to Winston Churchill, then First Lord of the Admiralty, by British Army Lt.-Col. Ernest Swinton. From this report, three possible terms emerged: "cistern", "motor-war car", and "tank". Apparently "tank" was chosen due to its linguistic simplicity. But perhaps the most compelling story comes from Winston Churchill's authoritative biography. To disguise the device, drawings were marked "water carriers for Russia." When it was pointed out that this might be shortened to "WCs for Russia," the drawings were changed to "water tanks for Russia." Eventually the weapon was just called a tank.