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3D (or 3-D) means three-dimensional, or having three dimensions. For example, a box is three-dimensional; it is solid, and not thin like a piece of paper. It has volume, a top and bottom, left and right (sides), as well as a front and back. You can turn the box around to view it from another side (called a face, surface or simply side).
3D graphics are used to make video games or animated movies. A lot of calculations are needed to make pictures seem three-dimensional on a screen. Modern computers usually have a GPU (graphics processing unit) to handle these calculations.
In the Cartesian coordinate system, length, width, and height are given letters (variables) to make them easier to write, or if a value is not known. Often X is width, Y is height, and Z is length. Another common 3D coordinate system uses spherical coordinates.