Focal distance, or length, of a lens or mirror

from The Collaborative International Dictionary of English v.0.48

Focal \Fo"cal\, a. [Cf. F. focal. See {Focus}.]
   Belonging to,or concerning, a focus; as, a focal point.
   [1913 Webster]

   {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.
      [1913 Webster]