The Collaborative International Dictionary of English v.0.48:

Bifocals \Bi"fo`cals\, 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]

WordNet (r) 3.0 (2006):

bifocals
    n 1: eyeglasses having two focal lengths, one for near vision
         and the other for far vision