A definition for derivative, definite integral, and indefinite integral (antiderivative) is necessary in understanding the fundamental theorem of calculus. The derivative can be thought of as measuring the change of the value of a variable with respect to another variable. The definite integral is the net area under the curve of a function and above the x-axis over an interval [a,b]. The indefinite integral (antiderivative) of a function f is another function F whose derivative is equal to the first function f.