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Characterized by movement of the lips or other speech organs without making audible sounds: subvocal speech.
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.
1. (Linguistics) (of speech or other sound) not voiced, involving movement of the lips or other speech organs but no vocalization
2. (Linguistics) involving the formation of words solely in the mind
Collins English Dictionary – Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014