extrados

This term is a specialized technical descriptor used primarily in architecture, civil engineering, and masonry. It describes the outer perimeter of a curved structure, specifically the upper surface of an arch. It is the direct geometric opposite of the intrados, which refers to the inner curve.

Because it is a precise architectural term, it is rarely encountered in casual conversation and is almost exclusively found in blueprints, structural reports, or historical analyses of building techniques. It functions as a formal noun to ensure there is no ambiguity between the interior and exterior surfaces of a structural curve.