The Collaborative International Dictionary
Focal distance or length of a lens or mirror
Focal \Fo"cal\, a. [Cf. F. focal. See Focus.] Belonging to,or concerning, a focus; as, a focal point.
Focal distance, or length, of a lens or mirror (Opt.), the distance of the focus from the surface of the lens or mirror, or more exactly, in the case of a lens, from its optical center.
Focal distance of a telescope, the distance of the image of an object from the object glass.