Macron

A macron ( ¯ ), from the Greek  (makrón), meaning "long", is a diacritic mark commonly placed above a single character or symbol (particularly vowels).Greve, Susan M. (2007).  Phonics for Dummies, p. 114.

Usage
In the romanization of Japanese, a letter with macron represents a long vowel.Nakao, Seigo. (1997).  Random House Japanese-English English-Japanese Dictionary, p. x.
For example, in the Hepburn romanization system
 kōtsū () means "traffic" 
 kotsu () means "bone" or "knack"

In the written Hawaiian language, the kahakō is shown by a macron.United Nations Group of Experts on Geographical Names, Twenty-third Session Vienna, "Working Paper No. 82, "US Board on Geographic Names: Collection and Dissemination of Indigenous Names," p. 3; excerpt, "An example of this has been the addition of the glottal stop (okina) and macron (kahako) to placenames of Hawaiian origin, which prior to 1995 had always been omitted. The BGN (Board on Geographic Names) staff, under the direction and guidance of the Hawaii State Geographic Names Authority, has been restoring systemically these marks to each Hawaiian name listed in GNIS (Geographic Names Information System)"; retrieved 2012-6-16.

In mathematics, a macron (or overline) is often placed above a symbol to refer to a complex conjugate, a sample mean and a logical negation.  
 Related pages 
Tilde
Vinculum (symbol)

 References 

Category:Alphabetic diacritics