Macron

n.

1.

(Pron.) A short, straight, horizontal mark [-], placed over vowels to denote that they are to be pronounced with a long sound; as, ā, in dāme; ē, in sēam, etc.

Noun

1.

macron - a diacritical mark (-) placed above a vowel to indicate a long sound

diacritic, diacritical mark