Word Study
triforium
CIDE DICTIONARY
triforium, n. [LL., fr. L. tri- (see Tri-) + foris, pl. fores, a door.].
The gallery or open space between the vaulting and the roof of the aisles of a church, often forming a rich arcade in the interior of the church, above the nave arches and below the clearstory windows. [1913 Webster]
OXFORD DICTIONARY
triforium, n. (pl. triforia) a gallery or arcade above the arches of the nave, choir, and transepts of a church.
Etymology
AL, of unkn. orig.
For further exploring for "triforium" in Webster Dictionary Online