smooth breathing

in written Greek, the mark (') placed over an initial vowel to show that in ancient Greek it was pronounced without a preceding (h) sound, or aspirate
the lack of aspiration thus indicated

Origin: transl. of L spiritus lenis

See smooth breathing in American Heritage Dictionary 4

noun

The symbol ( ) written over some initial vowels and diphthongs in ancient Greek to indicate that a word does not begin with the sound (h).
In ancient Greek, an initial vowel or diphthong not preceded by the sound (h).

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