Sqrt(-1)

Sqrt(-1) (Square Root from Negative One) is an imagery number, and a rogue root, playing against the rules of its former overlords. It is also a square root of the negative form of 1, which is an amazing number, most likely the best of all.
== Reason it's undefined ==
If x and y are <0, equation x * y forms a positive number. x^2 forms positive number as well, even if x < 0. Square root may be only calculated from positive numbers, since no number can be negative, after being raised to the power of 2. This number may be equal to at least 1i (i - imaginary unit)