Definition:Imaginary Unit/Also denoted as

Imaginary Unit: Also denoted as

The non-italicized $\mathrm i$ can also be seen.

Using the formal definition of complex numbers, the imaginary unit is the ordered pair $\tuple {0, 1}$.

When mathematics is applied to engineering, in particular electrical and electronic engineering, the symbol $j$ is usually used

This is because $i$ is the standard symbol used to denote the flow of electric current, and to use it also for $\sqrt {-1}$ would cause untold confusion.

In some mathematical traditions, the Greek symbol $\iota$ (iota) is used for $i$.

Sources

• 1990: H.A. Priestley: Introduction to Complex Analysis (revised ed.) ... (previous) ... (next): $1$ The complex plane: Complex numbers $\S 1.1$ Complex numbers and their representation
• 1998: David Nelson: The Penguin Dictionary of Mathematics (2nd ed.) ... (previous) ... (next): complex number
• 2008: David Nelson: The Penguin Dictionary of Mathematics (4th ed.) ... (previous) ... (next): complex number