barrel vaultn. A simple continuous vault, typically semicircular in cross section. Also called tunnel vault, wagon vault.American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.barrel vault n (Architecture) architect a vault in the form of a half cylinder. Also called: wagon vault or tunnel vault Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014bar?rel vault` n. a vault having the form of a very deep arch. Also called bar?rel roof`. [1840?50] bar?rel-vault`ed, adj. Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.