Heavy Tank was a Tank Variant that provided better armour protection as well as equal or greater firepower than tanks of lighter classes, at the cost of mobility and manoeuvrability and, particularly, expense.
The origins of the class date to World War I and the very first tanks; designed to operate in close concert with the infantry and facing both Artillery and the first dedicated anti-tank guns, early tanks had to have enough armor to allow them to survive on no man's land. As lighter tanks were introduced, the larger designs became known as heavies. The same basic role remained into World War II, with the British referring to them as Infantry Support indicating this close support role.
As tank combat became more common, especially tank-vs-tank, the heavies also became platforms to mount very powerful anti-tank guns, and the role of the heavies began to change. By the end of the war they were a primary class, used both for dealing with heavy fortifications as well as forcing its way through enemy tank formations. They were also known as breakthrough tanks, indicating their purpose of spearheading the attack. In spite of this, in practice they have been more useful in the defensive role than in the attack.
The emergence of the Main Battle Tank spelled the end of the heavy tank as a separate class, although a number of post-war examples were produced. These were generally gone by the 1960s.