The Collaborative International Dictionary
Undercroft \Un"der*croft\, n. [Under + Prov. E. croft a vault; cf. OD. krochte crypt, and E. crypt.] (Arch.) A subterranean room of any kind; esp., one under a church (see Crypt), or one used as a chapel or for any sacred purpose.
Douglas Harper's Etymology Dictionary
Wiktionary
n. A cellar or vaulted storage room.
Wikipedia
An undercroft is traditionally a cellar or storage room, often brick-lined and vaulted, and used for storage in buildings since medieval times. In modern usage, an undercroft is generally a ground (street-level) area which is relatively open to the sides, but covered by the building above.
Usage examples of "undercroft".
I stopped at Lincoln on the journey here, my Lord Silvester remembered the forgotten traveling trunk in the undercroft and decided to bring it to you.
Hiltrude went back up the stairs from the undercroft, and took care to spend her afternoon blamelessly among the women of the household, within sight every moment, and occupied with the proper affairs of the lady of the manor.
But if it were cut, said Waldo, it would go down into the orchestra pit, rather than the undercroft of the stage proper.
He refuged in the deep doorway to the undercroft, and stretched his ears for voices from above, and thought that he caught wordless murmurings, as though the aim was to keep everything of this night’s activities secret.
The village sat serenely in the meadows, just short of the foothills, the manor within its long stockade raised over an undercroft, and the small church close beside it.