Fissura is a term used to describe a narrow opening or a crack. Synonyms for this word include crevice, fissure, slit, gap, split, and cleft. Each of these words suggests a narrow opening or separation, but they may differ slightly in their connotations. For example, a crevice may be a narrow crack in a rock, while a fissure may refer to a more significant break in the earth's surface. A slit is often associated with a narrow cut or incision, while a gap suggests a space between two objects. Finally, a cleft refers to a deep, narrow opening, often found in rock formations.