The attacking of planetary targets with space-based weapons. In modern usage, the term usually indicates large-scale indiscriminate devastation as opposed to precision strikes. Planetary bombardment was a notorious strategy during the Second Interstellar Period, and is now forbidden by the Galapagos Accords
on the rules of war.
Having its origins with the ballistic missiles of the Nuclear Dark Days, planetary bombardment was commonly threatened during the Second Interstellar Period as a means of military expediency--why bother fighting an expensive and drawn-out space battle when you could just threaten to overwhelm a planet's defenses with immense swarms of missiles? A balance of terror akin to mutually assured destruction played out in many places during this time, and it was the failure of this deterrence which helped to precipitate the Interstellar Dark Ages