(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
tri·fo·ri·um (trī-fôr'ē-əm, -fōr'-) n.
pl.tri·fo·ri·a (-fôr'ē-ə, -fōr'-) Architecture A gallery of arches above the side-aisle vaulting in the nave of a church.
[Medieval Latin, a gallery in Canterbury Cathedral (later taken to mean "with three openings").]