Euler's Formula

Theorem
Let $z \in \C$ be a complex number.

Then:
 * $e^{i z} = \cos z + i \sin z$

where:
 * $e^{i z}$ denotes the complex exponential function
 * $\cos z$ denotes the complex cosine function
 * $\sin z$ denotes complex sine function
 * $i$ denotes the imaginary unit.

Also known as
Euler's Formula in this and its corollary form are also found referred to as Euler's Identities, but this term is also used for the specific example:
 * $e^{i \pi} + 1 = 0$

It is wise when referring to it by name, therefore, to ensure that the equation itself is also specified.