1) a natural cleft between body parts or in the substance of an organ: as

a) any of several clefts separating the lobes of the liver

b) any of various clefts between bones or parts of bones in the skull

c) any of the deep clefts of the brain; especially one of those located at points of elevation in the walls of the ventricles — compare SULCUS

2) a break or slit in tissue usually at the junction of skin and mucous membrane (fissure of the lip)
3) a linear developmental imperfection in the enamel of a tooth