From 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]