barrel vault

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A simple continuous vault, typically semicircular in cross section.

from The Century Dictionary.

• noun A plain, semicylindrical vault, much used by ancient architects, and employed generally by medieval builders before the reappearance of groined vaulting at the close of the eleventh century.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun architecture A simple roof having a curved, often semicircular cross section; used to span large distances in railway stations, churches, etc. Usually supported on columns.
• noun roofing A building profile featuring a rounded profile to the roof on the short axis, but with no angle change on a cut along the long axis.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the simplest form of vault; a single continuous arch