(in a church) the wall at the side of the nave, choir, or transept, corresponding to the space between the vaulting or ceiling and the roof of an aisle, often having a blind arcade or an opening in a gallery.
Origin: 1695–1705; < AL, special use of ML triforium kind of gallery, lit., something with three openings, equiv. to L tri-tri-+ for(is) opening, door+ -ium-ium