# Definition:Complex Number/Definition 1

## Definition

A complex number is a number in the form $a + b i$ or $a + i b$ where:

$a$ and $b$ are real numbers
$i$ is a square root of $-1$, that is, $i = \sqrt {-1}$.

The set of all complex numbers is denoted $\C$.

## Also see

• Results about complex numbers can be found here.