a symbol (') used in the writing of Greek to indicate that the initial vowel over which it is placed is unaspirated.
Compare rough breathing.
Origin of smooth breathing
First recorded in 1740–50
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
British Dictionary definitions for smooth breathing
(in Greek) the sign (᾿) placed over an initial vowel, indicating that (in ancient Greek) it was not pronounced with an hCompare rough breathing
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012