Yes, it is a function.
You agree that y = x^2 gives you a "U" shaped graph, right? And y = -x^2 turns that "U" curve upside down. In both parabolas, they're pointing vertically.
But these parabolas can point horizontally, too. Instead of "x" being the variable that's squared, it will be "y" this time.
x = y^2 will give you a horizontal parabola, and to prove this, if you were to solve for y (which would give plus or minus square root of x), the curve of y = sqrt(x) is the same exact one as x = y^2.
Graph for y = sqrt(x):
The only difference between x = y^2 and y = sqrt(x) is that the curve for x = y^2 works for the bottom half of the graph, as compared to the curve of y = sqrt(x), which does not.
