The Collaborative International Dictionary of English v.0.48:

Meniscus \Me*nis"cus\, n.; pl. L. Menisci (-s[imac]), E.
   Meniscuses. [NL., from Gr. mhni`skos, dim. of mh`nh the
   moon.]
   1. A crescent.
      [1913 Webster]

   2. (Opt.) A lens convex on one side and concave on the other.
      [1913 Webster]

   3. (Anat.) An interarticular synovial cartilage or membrane;
      esp., one of the intervertebral synovial disks in some
      parts of the vertebral column of birds.
      [1913 Webster]

   Converging meniscus, Diverging meniscus. See Lens.
      [1913 Webster]

WordNet (r) 3.0 (2006):

meniscus
    n 1: (anatomy) a disk of cartilage that serves as a cushion
         between the ends of bones that meet at a joint [syn:
         meniscus, semilunar cartilage]
    2: (optics) a lens that is concave on one side and convex on the
       other
    3: (physics) the curved upper surface of a nonturbulent liquid
       in a vertical tube