In anatomy, a<unk> is a a natural fissure in a structure.
Examples include:
Esophageal<unk>  the opening in the diaphragm through   which the esophagus passes from the thorax into the abdomen<unk> for greater petrosal nerve
Maxillary<unk> Semilunar<unk>
Hiatus 
Transpyloric plane
Gumma 
Megaduodenum
Nipple
Pharyngeal recess
Atom
Lentulo spiral