Oval (projective plane)
（重定向自Hyperoval）
In projective geometry an oval is a circle-like pointset (curve) in a plane that is defined by incidence propertes. The standard examples are the nondegenerate conics. However, a conic is only defined in a pappian plane, whereas an oval may exist in any type of projective plane. In the literature, there are many criteria which imply that an oval is a conic, but there are many examples, both infinite and finite, of ovals in pappian planes which are not conics.