TRIFORIUM
Etymology
Noun
triforium (plural triforia or triforiums)
(architecture) The gallery of arches above the side-aisle vaulting in the nave of a church.
Source: Wiktionary
Tri*fo"ri*um, n. Etym: [LL., fr. L. tri- (see Tri-) + foris, pl.
fores, a door.] (Arch.)
Definition: 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.
Source: Webster’s Unabridged Dictionary 1913 Edition