• (n.) A gallery in a church, where persons watched all night.
Ex·cu`bi·to'ri·um noun [ Late Latin excubitorium ; ex out + cubare , cubitum , to lie.] (Eccl. Antiq.) A gallery in a church, where persons watched all night. 
An excubitorium is a gallery or loft in a church where watch was kept at night on the eve of any great festival, and from which the great shrines were observed.
(in an ancient Roman city) a night watchman's post or sentry box.