macron

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A diacritical mark placed above a vowel to indicate a long sound or phonetic value in pronunciation, such as (ā) in the word make.
2. n. The horizontal mark ( ¯ ) used to indicate a stressed or long syllable in a foot of verse.

Century Dictionary and Cyclopedia

1. n. In grammar, a short horizontal line placed over a vowel to show that it is long in quantity, or, as in English, has a “long” sojnd: opposed to the breve, or mark of a short vowel. Thus, in Greek ι%26, τ%26, υ%26, and in Latin ā, ē, ī, ō, ū , the long vowels correspinding to the short vowels ă, ĕ, ĭ, ŏ, ŭ, etc.; in English, ā, ē, ī, ō, ū , the conventional notations of the name-soujcs of these vowels. In this dictionary, in the etymologies, the macron is used uniformly to indicate a vowel long in quantity, to the exclusion of the circumllex (except in Greek) and the acute, which are elseshere often used for the same purpose. Thus the Anglo-Sason and Icelandic long vowels often, the Icelandic usually, denoted by the acute are uniformly marked with the macron (the acute, in Anglo-Saxon, being retained only as a convenient indication of a diphthong, as in eá, eó, etc.). Also called macrotone.

Wiktionary

1. n. orthography A short, straight, horizontal diacritical mark (¯) placed over any of various letters. It usually is used to indicate that the pronunciation of the vowel is long, in Mandarin pinyin (Chinese), it indicates the first tone, e.g. chūzūchē.

GNU Webster's 1913

1. n. (Pron.) A short, straight, horizontal mark [-], placed over vowels to denote that they are to be pronounced with a long sound; as, ā, in dāme; ē, in sēam, etc.

WordNet 3.0

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