The Collaborative International Dictionary of English v.0.48:

Fornix \For"nix\, n.; pl. Fornices. [L., an arch.] (Anat.)
   (a) An arch or fold; as, the fornix, or vault, of the
       cranium; the fornix, or reflection, of the conjuctiva.
   (b) Esp., two longitudinal bands of white nervous tissue
       beneath the lateral ventricles of the brain.
       [1913 Webster]

WordNet (r) 3.0 (2006):

fornix
    n 1: generally any arch shaped structure (but often it refers to
         the arched roof of an anatomical space)
    2: an arched bundle of white fibers at the base of the brain by
       which the hippocampus of each hemisphere projects to the
       contralateral hippocampus and to the thalamus and mamillary
       bodies [syn: fornix, trigonum cerebrale]