Ovoid

Summary

An ovoid is a set of points of a projective space that looks like a sphere from the point of view of incidence. Every line meets it in at most two points, and at each of its points the tangent lines fill a hyperplane. Ovoids in three-dimensional space are the spatial analogue of ovals in a plane. They give Möbius planes and generalized quadrangles, and finite ovoids are a main topic of Galois geometry.