The sharp edge or salient angle formed by two surfaces meeting each other, whether plane or curved; -- applied particularly to the edges in moldings, and to the raised edges which separate the flutings in a Doric column.
X-shaped projection at intersection of two semi-circular (barrel) vaults, running diagonally from pier to pier. Because it takes its shape in response to the rest of the vault, it is geometrically indeterminate, and therefore aesthetically unsatisfactory in Medieval terms.
External angular intersection between two planar faces, or two curved faces (as in moldings), or between two flutes on a Doric column, or between a flute and the fillet on an Ionic or a Corinthian column.
Arris is an architectural term that describes the sharp edge formed by the intersection of two planes, such as the corner of a masonry unit; the junction between two planes of plaster or any intersection of divergent architectural details. Also the raised edges which separate the flutings in a Doric column.