Answer
Top half of an ellipsoid.
Work Step by Step
Trace in x=0: top half of an ellipse
Trace in y=0: top half of an ellipse
Traces when $z$ is below $0$: none (square roots are nonnegative).
Traces for z between 0 and 2: ellipses, $4x^{2}+y^{2}=k^{2}.$
Graph: top half of an ellipsoid.