1. A circular building; especially, one with a dome.
2. A large area with a high ceiling or a large round room; such as, in a hotel lobby or a government building (capitol) in the capital city of a state or country.

An example of a building with a rotunda is the white marble domed building in Washington, D.C., where the U.S. Congress meets.

A membrane-covered opening between the middle and inner ears in amphibians and mammals through which energy is sent after traveling in the membranous labyrinth: The fenestra rotunda is a small opening; especially, either of two window-like apertures in the medial wall of the middle ear.