Two-dimensional means having only length and width, but no depth or thickness. A drawing on paper is two-dimensional: you can measure how tall and wide it is, but it has no real thickness extending out toward you. A photograph, a painting, or an image on a computer screen are all two-dimensional.

Think of the difference between a picture of a cube and an actual wooden block. The picture is flat, existing only on the surface of the paper. The real block takes up space: you can hold it, walk around it, and see different sides. The block is three-dimensional because it has length, width, and depth.

Mathematicians and scientists often abbreviate two-dimensional as 2D and three-dimensional as 3D. In geometry class, you might draw 2D shapes like squares and circles, while 3D shapes include cubes and spheres.

The term can also describe something that lacks depth in a different way. When someone criticizes a movie character as two-dimensional, they mean the character feels flat and simple, without the complexity that real people have. A well-written character seems three-dimensional: they have contradictions, hidden depths, and realistic motivations that make them feel real.