Is Kusama part of Polkadot?

Excuse me, could you please clarify for me if Kusama is indeed a part of the Polkadot ecosystem? I've heard some people mentioning that they are interconnected, but I'm not entirely sure about the nature of their relationship. Could you elaborate on how Kusama relates to Polkadot, and if there are any specific functions or roles that Kusama plays within the Polkadot network? I'd appreciate any insights you can provide on this matter.