Fō'eal, a.

a. [Fr. focal. See FOCUS.] Belonging to, or concerning, a focus; as, a focal point.

Focal distance of a lens or mirror (Opt.), the distance of the focus from the center. — Focal distance of an ellipse, distance from the focus to the nearest extremity of the transverse axis. — Focal distance of a telescope, the distance of the image of an object from the object-glass.