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[=a]me; [=e], in s[=e]am, etc.

   Syn: macrotone.
        [1913 Webster]

WordNet (r) 3.0 (2006):

macron
    n 1: a diacritical mark (-) placed above a vowel to indicate a
         long sound