[see page source (this Web page) for HTML tags]

http://en.wikipedia.org/wiki/Overline

... in math, technically known as a macron ...
http://mathworld.wolfram.com/Macron.html

Examples:

A macron is a bar placed over a single symbol or character ... symbolizes ... the mean, a.k.a., the arithmetic mean....

A bar placed over multiple symbols or characters is called a vinculum.