Focus (geometry)

In geometry, focuses or foci (UK /ˈfoʊkaɪ/, US /ˈfoʊsaɪ/), singular focus, are special points with reference to which any of a variety of curves is constructed. For example, one or two foci can be used in defining conic sections, the four types of which are the circle, ellipse, parabola, and hyperbola. In addition, two foci are used to define the Cassini oval and the Cartesian oval, and more than two foci are used in defining an n-ellipse.