intrados

This term is a specialized architectural descriptor used primarily in engineering, masonry, and historical preservation. It describes the specific geometry of an arch's underside, contrasting directly with the extrados, which refers to the outer curve. Because it is a technical term of art, it is rarely encountered in casual conversation and is almost exclusively found in blueprints, structural reports, or architectural critiques.

In a professional context, the word carries a neutral, descriptive tone. It is used to pinpoint the exact location of structural issues, such as spalling or cracking, ensuring that engineers and architects are referring to the same physical surface of the vault or arch.