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 ]