The square root of -1 is known as i.
The 'i' is short for "imaginary unit". The square root of any negative number is an imaginary number. The number does not exist. Or to put it another way, it's not real. Infact, numbers like 2, 3, 894, are called "real numbers". A combination of real numbers and imaginary numbers such as 6 + i, or 8 + 4i are called "compl...