Three-dimensional means having length, width, and height. A drawing on paper is flat and two-dimensional: it has length and width but no thickness. But a box, a ball, or a person is three-dimensional because you can measure it in all three directions. Walk around a sculpture and you'll see different sides: that's what makes it three-dimensional.

We often shorten this to 3D. When you watch a 3D movie with special glasses, the images seem to pop out from the screen, creating the illusion of depth. Video game designers build three-dimensional worlds that players can explore from different angles, unlike old arcade games where characters could only move left, right, up, and down on a flat screen.

Everything you can actually pick up and hold is three-dimensional. Your pencil has length, width, and thickness. Your chair has all three dimensions. Even something as thin as a sheet of paper is technically three-dimensional, though its height is so small we barely notice it.

The phrase three-dimensional can also describe a character in a story who feels like a real person with complex thoughts and feelings, rather than someone simple and predictable. A three-dimensional character has depth, just like a three-dimensional object.