The line over a repeating decimal is called a vinculum. This symbol is placed over numbers appearing after a decimal point to indicate a numerical sequence that is repeating.
The vinculum has a second function in mathematics. It is placed over a series of algebraic terms to indicate that the terms should be considered as a group and is used in the same way that parenthesis might be used in an algebra problem.
When used with Roman numerals, the symbol is used to indicate that a numeral should be multiplied by 1,000. The vinculum was not used in Roman times, but it was used with Roman numerals during the Middle Ages.