Why is it called "3D"?
The name "3D" is a short-hand abbreviation for "three-dimensional," meaning that an image or object exists (or appears to exist) on three axes -- X, Y and Z. If you think of a piece of graph paper, the X axis runs horizontal across the page and the Y axis runs vertically. This makes up the basics of a two-dimensional (or 2D) image. If you were to put a pencil the center of that piece of the graph paper, the pencil would become the Z axis. You would then have enough axes to create a three dimensional, or 3D, image.