(say "E to the power of i pi, plus one, equals zero")
This equation, by Leonhard Euler (say "oiler"), 1707-1783, is widely regarded as the most beautiful in all mathematics. Called "Euler's identity", it relates the numbers 1, 0, e (2.71828..., the base of natural logarithms), π (3.141592..., the ratio of a circle's diameter to its circumference) and i (the square root of minus 1, the basis of the "imaginary" numbers) - numbers nobody expected to be related. And it unites the relations of addition, multiplication, exponention and equation, all in a few elegant symbols.
Here it is, plain and unadorned.