[17th Miss Regt]

In Euclidean geometry, yes. But in spherical (or hyperbolic) geometry circles are well defined and have circumferences greater than or less than 2*pi*radius.

[IronJack]

Right, and in non-Euclidean space, parallel lines actually intersect, and the angles in a triangle total less than or more than 180 degrees.

I'm talking planar geometry.