Why is benzene called "aromatic"? Why not "circular" instead?
Aromaticity is a electronic property of a molecule, not a description of its shape. An aromatic molecule contains a circular system of conjugated π bonds and p orbitals that contains 4n+2 electrons, where n is an integer.

Benzene is aromatic: it has 6 electrons in a circular conjugated system (4n+2=6, n=1).

Cyclooctatetraene is not aromatic: it appears to have a circular conjugated system, but it contains 8 electrons (4n+2=8 would require n=3/2). This molecule actually adopts a non-planar geometry to avoid antiaromaticity. 

Cyclohexane is not aromatic, even though it is circular: it does not have a conjugated electron system.