Macron 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.

Example:
How do I type a letter with a Macron and an acute accent?

Taxonomy
macron, long vowels, languages, vowel, tone, syllables, ü, lowercase, vowel length, free encyclopedia, mark long, Transcriptions, alphabet, Māori Language Commission, Character, Unicode, Nahuatl, diacritics.

Defination from the web:
macron, from the Greek μακρόv (makrón), meaning "long", is a diacritic placed above a vowel, but under a consonant (However the places switch) ... Some modern dictionaries of classical Greek and Latin, where the macron is sometimes used in conjunction with the breve. ...