Looks easy, but it's not...

imaginary numbers are the values of square roots of negitive integers, and are denoted by i. (as in this question)

We know, $i = \sqrt{-1}$

$\sqrt { -1 } :=i\quad \therefore \quad \sqrt { -1 } =i$

"An imaginary number is a complex number that can be written as a real number multiplied by the imaginary unit i which is defined by its property i^2 = −1." - Uno Ingard, K. (1988). "Chapter 2". Fundamentals of waves & oscillations. Cambridge University Press. p. 38. So, basically, imaginary numbers are the results of square roots of negative integers. And, are represented by the letter "i".