having the form of a circle

the building features a circular atrium

(of an argument) already containing an assumption of what is to be proved, and therefore fallacious

the reality of standard English rests on the circular argument that what is good is what good users use

(of a letter or advertisement) for distribution to a large number of people

a circular letter was sent asking for support

a letter or advertisement that is distributed to a large number of people

I received a circular from a building society