Cardassia Prime is a class M planet located in the Cardassian system of the Alpha Quadrant. It was the homeworld of the Cardassians as well as the capital of the Cardassian Union.
Cardassia Prime was located close to Bajor. (Star Trek: Deep Space 9)
After the Cardassian Union joined the Dominion in 2373, the planet became the Dominion's headquarters as they prosecuted their war in the Alpha Quadrant. (DS9: "By Inferno's Light")