A macron is a horizontal mark, usually written above a vowel or syllable, that shows the sound is long or metrically heavy in classical prosody. It is not the same as a stress mark. In poems based on quantity rather than stress, macrons help readers scan the meter and hear the difference between long and short syllables.
Example
The teacher marked the long syllables like this:
mater, linea, poeta
The macron shows which vowels should be read as long in scanning the verse.
Related Definitions
[n] a diacritical mark (-) placed above a vowel to indicate a long sound