rear admiraln.1. A commissioned rank in the US Navy or Coast Guard above captain and below vice admiral. The rank is split into rear admiral, upper half and rear admiral, lower half (formerly commodore).2. One who holds this rank.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.rear admiral n (Nautical Terms) an officer holding flag rank in any of certain navies, junior to a vice admiral Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014rear? ad?miral n. a commissioned officer in the U.S. Navy or Coast Guard ranking above a captain. [1580?90] Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.