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,

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

the quantity ab + 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