macron

n. horizontal line placed above a vowel to indicate a long sound (Phonetics)

A macron, from Greek  (makrón) meaning "long", is a diacritic ¯ placed over a vowel which was originally used to mark a long (i.e. heavy) syllable in Græco-Roman metrics, but has now been taken also to indicate that the vowel is long. The opposite is a breve ˘, used to indicate originally a short syllable and now also a short vowel. Distinctions between long and short vowels are usually phonemic. The sign to indicate a long vowel in the International Phonetic Alphabet is a modified triangular colon.
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Noun
1. a diacritical mark (-) placed above a vowel to indicate a long sound
(hypernym) diacritical mark, diacritic

n. macron, horizontal line placed above a vowel to indicate a long sound (Phonetics)

(n.)
A short, straight, horizontal mark [-], placed over vowels to denote that they are to be pronounced with a long sound; as, a, in dame; /, in s/am, etc.