cavity

A cavity is a hollow space. In a tooth it is a hole, and in a wall it is the gap between two skins of brick.

Dentistry gives most people their first cavity, and getting one filled is the standard treatment. Building uses the word constantly, since a cavity wall has an air gap that can be filled with insulation. Anatomy has several, including the chest cavity and the nasal cavity. The word is neutral and technical, which is why doctors and builders both like it, and the adjective is cavernous for a very large one.