Hypertext Webster Gateway: "macron"
From Webster's Revised Unabridged Dictionary (1913) (web1913)
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.
From WordNet (r) 1.7 (wn)
macron
n : a diacritical mark (-) placed above a vowel to indicate a
long sound
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