Vinculum

A vinculum is a horizontal line placed over a mathematical expression, used to indicate that it is to be considered a group.

Examples of its use include the case of a group of infinitely repeating digits, for example,

$\frac{1}{3} = 0.333333\dots = 0.\overline{3}$

It is also used in the notation of a radical to indicate the radicand whose root is being indicated. In the case of

$\sqrt[n]{ab+2}$

the quantity $a b + 2$ is the radicand, and thus has a vinculum over it.

The vinculum is also sometimes used in Boolean algebra, where it serves to indicate a group of expressions whose logical result is to be NOTed, as in

$\overline{AB}$