The curves obtained by slicing the cone with a plane not passing through the vertex are called **conic sections** or conics.

A conic is the locus of a point which moves in a plane, so that its distance from a fixed point bears a constant ratio to its distance from a fixed straight line.

The fixed point is called **focus**, the fixed straight line is called **directrix**, and the constant ratio is called **eccentricity**.

**FP/PM = e**