The term "imaginary" for √-1 was first used by René Descartes in 1637. The term was chosen because most people considered these numbers to be fictitious or useless. 

The notation 'i' for √-1 was introduced later by the mathematician Leonhard Euler in the 18th century.

 is the principal square root of –1. This also means that 

. This video explains how this unit works with some examples.

The imaginary unit i is the principal square root of –1. This also means that i squared equals –1. This video explains how this unit works with some examples. 

The imaginary unit i is the principal square root of minus 1

A real number is any value that can be represented on a number line. Examples of real numbers are integers, decimals, fractions, square roots and the number π

A complex number, z, is a number that can be written in the form

Introduction to Complex Numbers

What is the Imaginary Unit, i?