Noun
A subterranean room of any kind; esp., one under a church (see Crypt), or one used as a chapel or for any sacred purpose.
Source: Webster's dictionarySome of the lower parts of the monastic dormitory, an extension of the South Transept, survive in the Norman undercroft of the Great School, including a door said to come from the previous Saxon abbey. Source: Internet
The noise turned out to be the then-tenant's widow, who was clearing out the undercroft directly beneath the House of Lords—the room where the plotters eventually stored the gunpowder. Source: Internet
The only traces of St. Dunstan's monastery remaining are round arches and massive supporting columns of the undercroft and the Pyx Chamber. Source: Internet