The Collaborative International Dictionary of English v.0.48:

Fissure \Fis"sure\, n. [L. fissura, fr. findere, fissum, to
   cleave, split; akin to E. bite: cf. F. fissure.]
   A narrow opening, made by the parting of any substance; a
   cleft; as, the fissure of a rock.
   [1913 Webster]

   Cerebral fissures (Anat.), the furrows or clefts by which
      the surface of the cerebrum is divided; esp., the furrows
      first formed by the infolding of the whole wall of the
      cerebrum.

   Fissure needle (Surg.), a spiral needle for catching
      together the gaping lips of wounds. --Knight.

   Fissure of rolando (Anat.), the furrow separating the
      frontal from the parietal lobe in the cerebrum.

   Fissure of Sylvius (Anat.), a deep cerebral fissure
      separating the frontal from the temporal lobe. See Illust.
      under Brain.

   Fissure vein (Mining), a crack in the earth's surface
      filled with mineral matter. --Raymond.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Fissure \Fis"sure\, v. t.
   To cleave; to divide; to crack or fracture.
   [1913 Webster]

WordNet (r) 3.0 (2006):

fissure
    n 1: a long narrow depression in a surface [syn: crevice,
         cranny, crack, fissure, chap]
    2: a long narrow opening [syn: crack, cleft, crevice,
       fissure, scissure]
    3: (anatomy) a long narrow slit or groove that divides an organ
       into lobes
    v 1: break into fissures or fine cracks