Complex numbers are a mathematical extension of real numbers which introduce the imaginary unit denoted by "i". The imaginary unit is defined as the square root of -1, represented as \$i^2 = -1\$. A complex number is written in the format of
"a + bi".

Explanation: The given image shows "a" is the real part, representing the horizontal component. "bi" is the imaginary part, representing the vertical component multiplied by the imaginary unit "i".