Circular means shaped like a circle or moving in a circle. A coin is circular, with no corners or edges, just a smooth round shape all the way around. When ice skaters perform a circular pattern on the ice, they trace a path that curves back to where they started.

The word also describes reasoning or arguments that don't actually prove anything because they end up back where they began. If someone says “This book is popular because everyone likes it, and everyone likes it because it's popular,” that's circular reasoning. They haven't explained why it's popular, they've just restated the same idea in different words. It's like a snake trying to eat its own tail: it goes round and round without getting anywhere.

You might also hear about a circular in the mail, which is an advertisement or announcement meant to be passed around to many people.

The key idea in all these uses is that something comes back around to its starting point, whether that's a physical shape, a flawed argument, or information making the rounds through a community.