Answer
The given equation does not define $y$ as a function of $x$.
Work Step by Step
An equation defines $y$ as a function of $x$ if for every value of $x$, the equation gives only one corresponding value of $y$.
Note that the given equation gives two different y-values for most values of $x$.
Example:
When $x=0$:
$y^2=4-x^2
\\y^2=4-0^2
\\y^2=4
\\y = \pm \sqrt{4}
\\y=\pm 2$
Therefore, the given equation does not define $y$ as a function of $x$.
