A tank's cupola is a raised, armored structure on the roof of a [[tank]]/armored vehicle or [[tank turret]] that often carries observation devices and has a [[hatch]]. 

Tanks of WW2 often had large cupolas on their turrets to allow space for periscopes or windows to give a crew member, often the commander, a much better view outside the vehicle. Usually they were also the most logical place for the commander's hatch, as it would allow them to stand partially outside the vehicle for a better view or to operate a top-mounted [[machine gun]]. As technology progressed and allowed for smaller periscopes and later cameras, cupolas became shorter and smaller, to the point that some modern vehicles don't have any at all.
tank cupola