An antiderivative, also called a primitive, as its name implies, is the opposite of a derivative in calculus. That is, it is a function for which the given function is the derivative. It is important to note that there are an infinite number of antiderivatives for every function, since constants disappear during differentiation. For this reason, an arbitrary constant is often attached to the antiderivative, making an indefinite integral (not to be confused with an improper integral).