If something is circular, it has a round shape. You might take a circular route on your jog, running all the way around the park.
You can build a circular fence around your garden, or pipe frosting flowers in a circular design on top of a birthday cake. Look at either from above, and they form circles. In logic, a circular argument is one that ends up exactly where it started — you assume something, rather than proving it. A paper advertisement can also be called a circular, from the idea that it's distributed to a certain circle, or group, of people.