The Fundamental Theorem of Algebra (1702)

Theorem: Any polynomial of degree n has n roots, some of which may be repeated. If any root is repeated, the number of repetitions is referred to as the multiplicity of the root. The Fundamental Theorem can be restated to say that the sum of multiplicities for all roots of an nth degree polynomial is equal to n. The roots may be either real or complex.

Author: Karl Friedrich Gauss first proved the theorem around 1798.

Importance: The proof of the theorem made it absolutely clear that there are some numbers that can't be seen, in the normal sense of identifying roots at the points where their graphs crossed the x-axis. This was one of the first results in what would eventually become known as complex number theory.