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.

In Greco-Roman metrics and in the description of the metrics of other literatures, the macron was introduced and is still widely used to mark a long (i.e., heavy) syllable. Even the best and relatively recent classical Greek and Latin dictionaries are still only concerned with indicating the length (i.e., weight) of syllables; that is why most still do not indicate the length of vowels in syllables that are otherwise metrically determined. Though many textbooks about ancient Rome and Greece employ the macron, it was not actually used at that time.