Differentiation is the process of determining the slope of a function f(x) from its values; or if the variable is time t, finding the velocity of an object at a given time from  information about its position. Any function f which looks like a straight line sufficiently close to the point x can be differentiated at x and the result is the slope of that straight line.

This slope is called the derivative of f and is denoted by f'(x), or or sometimes by D_x f(x).