Ceva's theorem

Etymology
Named after Italian mathematician (1647-1734), to whom the discovery of the theorem is often attributed.

Proper noun

 * 1)  A theorem about triangles in plane geometry, regards the ratio of the side lengths of a triangle divided by cevians.

Usage notes
In mathematical terms, the theorem states: let D, E, and F, be points on the sides BC, CA, and AB of a triangle (possibly extended). Then AD, BE, and CF are concurrent if and only if $$\frac{AF}{FB} \cdot \frac{BD}{DC} \cdot \frac{CE}{EA} = 1.$$