An apse is an architectural structure in a semi-circular or polygonal shape. It is located in churches at the end of the main nave or, if the church has a central plan, in front of the entrance, where the main altar is usually located.
The apse is covered by a vault, called the apsidal vault, which generally has the shape of a semi-dome.
The name comes from the Latin apsis or from the Greek hapsis, meaning “arch” or “vault”.