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

Moby Thesaurus II by Grady Ward, 1.0:

209 Moby Thesaurus words for "fissure":
   abysm, abyss, amputate, arachnoid, arbor vitae, archipallium,
   arroyo, ax, between brain, bisect, box canyon, brain stem, breach,
   break, break open, break up, breakage, broach, burst, bust,
   butcher, caesura, canyon, carve, cavity, cerebellar hemispheres,
   cerebellum, cerebral cortex, cerebrospinal fluid, cerebrum,
   cessation, chap, chasm, check, chimney, chink, chip, chop, cleave,
   cleft, cleuch, clough, col, come apart, come unstuck, convolution,
   corpus callosum, corpus striatum, coulee, couloir, crack, cranny,
   crevasse, crevice, cut, cut away, cut in two, cut off, cut open,
   cwm, defile, dell, dichotomize, diencephalon, dike, disintegrate,
   dispart, dissever, ditch, divaricate, divide, donga, draw,
   dura mater, endbrain, excavation, excise, exfoliate,
   fall to pieces, fault, flaw, flume, fly open, folia, forebrain,
   fornix, fracture, frontal lobe, furrow, gap, gape, gash, give away,
   give way, glial cells, globus pallidus, gorge, gray matter, groove,
   gulch, gulf, gully, gyrus, hack, halve, hew, hiatus, hindbrain,
   hippocampus, hole, hypothalamus, incise, incision, interim,
   intermission, interruption, interval, jigsaw, joint, kloof, lacuna,
   lance, lay open, leak, lenticular nucleus, letup, limbic lobe,
   little brain, lobe, lull, mantle, medulla oblongata, meninges,
   mesencephalon, metencephalon, midbrain, moat, myelencephalon,
   neopallium, notch, nullah, occipital lobe, ope, open, open up,
   opening, optic chiasm, pallium, pare, parietal lobe, part, pass,
   passage, pause, pia mater, pineal body, pituitary body, pons,
   prune, ravine, rend, rent, reticular system, rhombencephalon, rift,
   rime, rip, rive, rupture, saw, scale, schism, scissor, scissure,
   seam, separate, sever, slash, slice, slit, slot, snap, snip,
   splinter, split, spread, spread out, spring a leak, spring open,
   start, subthalamus, sunder, suspension, swing open, tap, tear,
   tear open, telencephalon, temporal lobe, thalamus, throw open,
   trench, valley, ventricle, vermis, void, wadi, white matter,
   whittle