A conic section is defined as the locus of all points where the distance from the point to the focus and the point to a straight line known as the directrix is a constant ratio. This ratio is known as the eccentricity of the section.

- If the ratio is 1, the section is known as a parabola.
- If the ratio is less than one, the section is an ellipse. A circle is a special case of an ellipse.
- If the ratio is greater than one, the section is hyperbola.

All conic sections can be formed by taking the intersection of a cone and a plane, hence the name.