(in Greek) the sign (῾) placed over an initial letter, or a second letter if the word begins with a diphthong, indicating that (in ancient Greek) it was pronounced with an h
Compare smooth breathing
rough breathing in American English
1.
in written Greek, the mark (ʼ) placed over an initial vowel or ρ (rho) to show that in ancient Greek it was pronounced with a preceding (h) sound, or aspirate