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.

Source: Webster's Revised Unabridged Dictionary (1913)