A line above a mathematical symbol or expression is used in a number of different contexts in mathematics. When placed over is single character or symbol, it is known as a macron; when placed over a larger expression, it is known as a vinculum.
Overline
See also
Macron, VinculumExplore with Wolfram|Alpha
Cite this as:
Weisstein, Eric W. "Overline." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/Overline.html