a. [ Pref. bi-+ focal. ] Having two foci, as some spectacle lenses. [ Webster 1913 Suppl. ]
n. [ Pref. bi-+ focal. ] a bifocal lens. [ PJC ]
n. pl. [ Pref. bi-+ focal. ] eyeglasses whose lenses have two foci, allowing the wearer to see both far and nearby objects clearly. The lenses are partitioned horizontally, the upper and lower parts having different focal lengths. [ PJC ]
a. (Math.) Having the same foci;
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.),
Focal distance of a telescope,
n. The act of focalizing or bringing to a focus, or the state of being focalized. [ 1913 Webster ]
v. t.
Light is focalized in the eye, sound in the ear. De Quincey. [ 1913 Webster ]
a. [ Pari- + focal. ] (Opt.) With the lower focal points all in the same plane; -- said of sets of eyepieces so mounted that they may be interchanged without varying the focus of the instrument (as a microscope or telescope) with which they are used. “the convenience of a microscope with parfocal objective lenses” [ Webster 1913 Suppl. ]