is a circle, for a parabola. (The plane must not meet the vertex of the cone.)

The linear eccentricity of an ellipse or hyperbola, denoted (or sometimes or ), is the distance between its center and either of its two foci. The eccentricity can be defined as the ratio of the linear eccentricity to the semimajor axis : that is, (lacking a center, the linear eccentricity for parabolas is not defined). A parabola can be treated as a limiting case