The Fundamental Theorem of Calculus (1660)

Theorem: This theorem proved what had long been suspected, that under certain conditions of continuity and differentiability, the operations of differentiation and integration are inverse operations. If f(x) is a differentiable function, and F(x) is any antiderivative of f(x), then the derivative of F(x) is f(x), and the integral of the first derivative of f(x) is f(x) + c, where c is an arbitrary constant.

Author: As is so often the case, Newton routinely gets credit for the FTC, although many historical documents seem to establish that it was first proven by Isaac Barrow, around 1655.

Importance: The theorem solidified the perceived relationship between integration and differentiation, clearing the way for the rapid development of Calculus concepts throughout the rest of the 17th century, and into the 18th.