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A preposition used as a mark of noble rank before a surname, as German von in Wernher von Braun and French de in Simone de Beauvoir
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.
a preposition, such as French de or German von, occurring as part of a title or surname: Marquis de Sade.
Collins English Dictionary – Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014