Definition of "macron"

We say such words belong to a Long family, because of the long macron hat the vowels wear. Like the other twin o's, one hat is enough for both, for these twins are always found together.

1889, Rebecca S[mith] Pollard, A Complete Manual: Pollard’s Synthetic Method of Reading and Spelling. Designed to Accompany Synthetic Readers and Spellers, New York, N.Y.: American Book Company, page 75

A macron is a mark placed above a long vowel to mark its quantity.

1974, Floyd L. Moreland, Rita M. Fleischer, Latin: An Intensive Course, University of California Press, page 13

All vowels are pronounced short unless marked with a ¯ (macron) over them. So observe different vowel length of ‘i ’ in, e.g., fīlia, etc. It may be helpful, but is not essential, to mark macra in your exercises.

1986, Peter V. Jones, Keith C. Sidwell, “Introduction”, in Reading Latin, volumes 1 (Grammar, Vocabulary and Exercises), Cambridge: Cambridge University Press, page 2, general note 1