Complex number

A complex number is a number composed of two parts - a real component and and imaginary component, of the form $a+bi$, where a and b are real numbers and i is the square root of negative 1.

Whereas the real numbers can be represented as all the possible points on an infinitely extended number line, to represent all the complex numbers requires the use of a two dimensional coordinate system, usually with the real components on the horizontal axis (the abscissa) and the imaginary components on the vertical axis (the ordinate).