Euler's Formula

Theorem

 * $e^{i \theta} = \cos \theta + i \sin \theta$

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

Thus the complex exponential function is defined in terms of standard trigonometric functions.

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.