Vinculum

A vinculum is a horizontal line placed over a mathematical expression to show that the enclosed terms are to be treated as a single grouped unit. It appears as the bar over the radicand in a square-root or nth-root symbol, over a fraction's numerator and denominator in some traditional notations, and over a sequence of repeating decimal digits to indicate that the digits recur indefinitely. Historically the vinculum also served as an early substitute for parentheses in algebraic notation before bracket symbols became standard.