Basic Information
For such an iconic piece of equipment, the tank, like the sword, can prove remarkably hard to define.
The first tanks were the lumbering, lozenge shaped things that the British army deployed in the muddy chaos of WW1's western front - and which picked up their incidental name as part of a scheme to disguise them as water carriers in the leadup to their combat debut. However, nothing that looks like that can be found on a modern battlefield. The modern tank is at least as much descended from the French FT-17 … a light, late war version that they came to after at least one dreadful false start. This design was disputed in various ways into the early years of WW2, but seems to have since solidified into an armoured, tracked hull with a single, closed and armoured turret carrying an ordnance class weapon in a 360' traverse1. WW2 also established that a tank functions best with a undistracted commander (whose job is to maintain awareness of the vehicle's environment, direct its movement and identify and prioritise targets) and a gunner who can focus on operating the gun (and any secondary armament if required) … which usually meant a fourth crewman to act as loader. Fourth because driving a tank has always been a full time job.
For the avoidance of doubt, if it is wheeled, it is not a tank2, if it carries passengers, it is not a tank3, if it does not have a turret, it is not a tank4 and if it doesn't have an ordnance class main armament it isn't a tank either5.
If the design question was complicated to resolve, then the role of the tank was even more so. The apparent lesson of WW1 was that the tank was an infantry support weapon, designed to provide cover and support fire to advancing infantry. Others considered that a tank had a specific purpose of reducing enemy strongpoints and, where required, destroying enemy tanks. More astute minds noticed that a tank, if properly designed and supported, could instead be used to break through enemy lines and run riot in his rear area. Some powers (notably the French) opted mainly for infantry support, others, such as the British, divided up effort between breakthrough ("cruiser") tanks and infantry tanks whilst still others (such as the Germans) neglected infantry support in favour of breakthrough. The tank killing and strongpoint reducing role was given to a separate subspecies of "heavy" tanks and WW2 was used to test the competing theories. On balance, the breakthrough role won but the fast, lightly armoured (and often lightly armed) breakthrough tanks of the early war were found to be too vulnerable to use in their intended role and so the final evolution (to date) was the modern "Main Battle Tank" (or MBT) - which is traditionally the heaviest combination of gun and armour that the user can make and still keep mobile enough for the breakthrough role. The MBT should, ideally, be designed to engage and defeat those enemy designs that it is likely to face, but should also be capable of effectively engaging enemy ordnance, infantry and lighter vehicles.