P9C-372 was a planet in the Milky Way galaxy that was once visited by the Ancients, during which time they deposited a Stargate on the planet. The planet was also homeworld to a race of intelligent electronic beings, known to the Tau'ri as Entity. The planet's architecture was not similar to that of any other race known to exist in the Milky Way.
In the aftermath of the Entity possessing Major Samantha Carter, Major General George Hammond ordered that the planet be locked out of the SGC's Dialing computer, resulting in the planet in question being declared off-limits while also ensuring that the SGC would never visit it ever again. (SG1: "Entity")