Macron

A macron ( ¯ ), from the Greek ("makrón"), meaning "long", is a diacritic mark commonly placed above a single character or symbol (particularly vowels).
Usage.
In the romanization of Japanese, a letter with macron represents a long vowel.
In the written Hawaiian language, the "kahakō" is shown by a macron.
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.