Answer
The equation DOES NOT define $y$ as a function of $x$.
Work Step by Step
Solve for $y$ in terms of $x$ to have:
$\pm \sqrt{x} = y
\\y=\pm \sqrt{x}$
The last equation is a rule that gives two values of $y$ for each valid value of $x$ (except x=0).
Thus, it does not define $y$ as a function of $x$