What architectural feature describes a continuously curved roof over a polygonal or circular plan?

A dome is defined as a structural feature that consists of a continuously curved roof, often taking the shape of a hemisphere. This architectural element can cover polygonal or circular plans and is recognized for its ability to create large, open interior spaces without the need for numerous supporting columns. Domes have been utilized in a variety of architectural styles throughout history, symbolizing stability and grandeur, and are featured in many significant buildings across cultures, such as churches, mosques, and government edifices.

In contrast, the engaged column refers to a column that is attached to a wall, providing decorative support but not embodying the continuity of curvature found in domes. The term exedra typically describes a semicircular or polygonal recess in a building, often used in classical architecture but does not encapsulate the concept of a curved roof. The frigidarium is related to Roman baths, depicting the cold room but is not a description of a roof structure. Therefore, the dome stands out as the fitting answer due to its defining characteristics and architectural significance.