The saddle surface is a hyperbolic paraboloid, named after it looks like a saddle.

The standard equation is x 2/a 2-y 2/b 2 = 2z (a, b >；; 0)

Z=xy mentioned above is of course a saddle surface, but it is not a standard equation. It can also be changed to the form of x 2/a 2-y 2/b 2 = 2z (a, b > 0).