Answer
$\text{not a function}$
Work Step by Step
For an equation to be a function: any input $x$ will yield only one output $y$
We start by solving the equation for $y$
Taking the square root of both sides:
$$y = \pm \sqrt{4-x^2}$$
If $x=1$
$$y = \pm \sqrt{4-(1)^2} = \pm \sqrt{3}$$
As there are two different outputs resulting from the same input, the equation isn't a function
