Source : Webster's Revised Unabridged Dictionary (1913)

Macron \Ma"cron\, n. [NL., fr. Gr. ? long.] (Pron.)
   A short, straight, horizontal mark [-], placed over vowels to
   denote that they are to be pronounced with a long sound; as,
   [=a], in d[=a]me; [=e], in s[=e]am, etc.

Source : WordNet®

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