As it was explained to me multiple times, at length
on the long and treacherous road to my Bachelor of Mathematics degree
'i' is not the square root of negative one. The square root of negative one is undefined, as the square root operation cannot be applied to numbers less than zero.
'i' is defined as a number with the property
i 2 = -1
Calling it imaginary and saying it's the square root of negative one really annoyed profs at the University of Waterloo, who clearly needed to get out more often.