To determine whether the given relation (the semicircle) is a function, we can apply the vertical line test. According to this test, if any vertical line intersects the graph of the relation more than once, then the relation is not a function.
In this case, the semicircle is defined by points above the x-axis and will be intersected by vertical lines in the upper half of the coordinate plane. Since for some x-values (specifically those corresponding to points within the semicircle's horizontal extent) there are two y-values (the upper and lower edges, for the negative y-values and positive y-values you would encounter in a full circle), a vertical line would indeed cross the semicircle at two points.
Given that the semicircle passes through points like (0, 6) and (0, -6), it can be inferred that for certain x-coordinates, multiple y coordinates exist. Thus, we conclude:
No, the given relation is not a function.