[1] : The Collaborative International Dictionary of English v.0.48

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=ame; =e, in s=eam, etc.

   syn: macrotone.
        1913 webster

[2] : WordNet (r) 2.0

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

Macron >> M Words
Macron, definition of term: Macron
macron_pag1.html