A derivative in calculus is the rate of change of a function with respect to another quantity or variable. It is also known as the differential. The derivative of a function can be interpreted geometrically as the slope of the graph of the function or, more precisely, as the slope of the tangent line at a point. The derivative is often described as the "instantaneous rate of change," which is the ratio of the instantaneous change in the dependent variable to that of the independent variable. The derivative of a function of a single variable at a chosen input value, when it exists, is the slope of the tangent line to the graph of the function at that point. Differentiation is the process of finding the derivative of a function. The derivative of a constant is always zero. The derivative of a monomial can be found using the power rule, which states that the derivative of x^n is n*x^(n-1) .