The Theodorus spiral, also known as the Einstein spiral, Pythagorean spiral, square root spiral, or--to contrast it with certain continuous analogs--the discrete spiral of Theodorus, is a discrete spiral formed by connecting the ends of radial spokes corresponding to the hypotenuses of a sequence of adjoining right triangles. The initial spoke is of length , the next spoke is of length , etc., and each segment of the spiral (corresponding to the outer leg of a triangle) has unit length.
The slope of a continuous analog of the discrete Theodorus spiral due to Davis (1993) at the point is sometimes known as Theodorus's constant.