In StarCraft II, the Siege Tank is the Terran unit that can transform between combat and siege modes.
In Tank mode, the unit can move and fire at nearby targets. In Siege mode, it becomes stationary and gains long-range artillery fire with powerful area damage. This transformation gives Terran players a way to defend positions, control approaches, and punish groups of enemy units.
The Siege Tank is often confused with the Viking because the Viking also changes form, but the Viking switches between an air-combat mode and a ground-combat mode. The Thor is a large Terran walker, and the Cyclone is a mobile vehicle with a lock-on weapon; neither uses the Siege Tank's two modes.
Siege Tanks require careful positioning because they cannot move while deployed. Their splash damage can also harm friendly units, so successful use depends on spacing and battlefield awareness. The unit has been a recognizable Terran battlefield tool since the original StarCraft era.