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