Fundamental Theorem of Calculus is a theorem that links the concept of the derivative of a function with the concept of the integral. This theorem really connected the relationship of both concepts.
first fundamental theorem of calculus: the definite integration of a function is related to its antiderivative, and can be reversed by differentiation
second fundamental theorem of calculus: the definite integral of a function can be computed by using any one of its infinitely many antiderivatives