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 movies are films that give the illusion of three-dimensional solidity, usually with the help of special viewing devices (glasses worn by viewers).