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A vinculum is a horizontal line placed over a mathematical expression, used to indicate that it is to be considered a group. Vinculum is Latin for "bond", "fetter", "chain", or "tie", which is roughly suggestive of some of the uses of the symbol.
Examples of its use include the case of a group of infinitely repeating digits, for example,
It is also used in common arithmetic to denote that the numerator is being divided by the denominator as a whole group.
It is also used in the notation of a radical to indicate the radicand whose root is being indicated. In the next case, the quantity is the radicand, and thus has a vinculum over it.
It is also used to show the repeating terms in a periodic continued fraction. Quadratic irrational numbers are the only numbers that have these.
The vinculum is also sometimes used in Boolean algebra, where it serves to indicate a group of expressions whose logical result is to be negated, as in
The vinculum should not be confused with a similar-looking vector notation, e.g. "vector from A to B", or "vector named a".