Paraboloid

A paraboloid is a three-dimensional shape that has two terms of x, y, or z that are squared and one term of x, y, or z that is not. Examples of equations that describe paraboloids are:

ax² + by² = z
or az² - by² = x

When both squared terms are positive, like in the first equation, the paraboloid is an elliptical paraboloid (shaped like a roundish, eggish blob).

When one squared term is negative and one is positive, like in the second equation, the paraboloid is a hyperbolic paraboloid (shaped like a saddle).