Orbital bombardment, also known as an orbital strike, was a military strategy, where huge star cruisers (such as Star Destroyers) would fire their main cannons to the surface of a planet, causing heavy damage to the enemy. This strategy was used mostly to wipe targets that were in close proximity, like regular ground troops, or was used to easily deal with large heavy targets. However, the tactic was imprecise, especially if the target was moving.
While orbital bombardment could be used to devastate a planet, like Base Delta Zero, it was more commonly used for military strikes to decimate a target area, and not an entire planet. Base Gamma One was a code in the Imperial Navy for a bombardment.