Macron

A macron, from the Greek μακρόν (makrón), meaning "long", is a diacritic placed above a vowel (and, more rarely, under or above a consonant). It was originally used to mark a long or heavy syllable in Greco-Roman metrics, but now marks a long vowel. In the International Phonetic Alphabet the macron is used to indicate mid tone; the sign for a long vowel is a modified triangular colon ⟨ː⟩.

The opposite is the breve ⟨˘⟩, which marks a short or light syllable or a short vowel.