what is the square root of -1

no I do not think so.

by the convention,      $$\sqrt{a\; number}\ge 0$$

for instance

$$\sqrt{25}=+5$$

even though    

   $$5^2=25\quad and\quad (-5)^2=25$$

$$\sqrt{-1}=i$$

i is NOT a real number it is an imaginary number.  It is the basic unit of the imaginary number system :)

actually,

the square root of -1 is ±1i