The notation by which the root is expressed, is the mark {\displaystyle {\sqrt {}}} called a radical, placed over the letter, with an exponent to the left indicating the order of the root.
1845, Dionysius Lardner, “Algebra”, in Edward Smedley, Hugh James Rose, Henry John Rose, editors, Encyclopædia Metropolitana; or, Universal Dictionary of Knowledge, on an Original Plan: Comprising the Twofold Advantage of a Philosophical and an Alphabetical Arrangement, with Appropriate Engravings, volume I (Pure Sciences, volume 1), London: B. Fellowes [et al.], page 534