Hiatus: An opening, as in the diaphragm. When there is an unusually wide opening in the diaphragm, there can be a hiatus (or hiatal) hernia.
In Latin an hiatus is "an opening, gaping mouth, or chasm." It is derived from the verb "hiare" meaning "to gape or yawn." Therefore, a "hiatus" is a gaping opening in anything, such as the opening in a cliff leading into a cave or a break in the conversation in which nothing happens or is said.