Phase shift is a term used in mathematics and physics to describe a horizontal shift of a periodic function, such as a sine or cosine wave, from its usual position. It is measured as the distance the function is shifted horizontally, either to the left or to the right, from its original position. The phase shift can be positive or negative, depending on the direction of the shift from the origin. In trigonometry, a phase shift is caused by adding or subtracting a certain value from the x-value in a function). The phase shift formula for a sine curve is y = A sin(B(x + C)) + D, where C/B represents the phase shift.