Merriam-Webster Collegiate Dictionary - fissure

 I. noun  Etymology: Middle English, from Middle French, from Latin fissura, from fissus  Date: 14th century  1. a narrow opening or crack of considerable length and depth usually occurring from some breaking or parting  2.  a. a natural cleft between body parts or in the substance of an organ  b. a break or slit in tissue usually at the junction of skin and mucous membrane  3. a separation or disagreement in thought or viewpoint ; schism ~s in a political party  II. verb  (~d; fissuring)  Date: 1656  transitive verb to break into ~s ; cleave  intransitive verb crack, divide