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The prefix a- or an- before vowels, used in Greek and in English words borrowed from Greek to express absence or negation.
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.
(Linguistics) (in Greek grammar) the letter alpha (or an- before vowels) used as a negative or privative prefix. It appears in English words derived from Greek, as in atheist, anaesthetic
Collins English Dictionary – Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014
the prefix a- or, before a vowel, an-, used in Greek and English to express negation or absence.
Random House Kernerman Webster's College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.