A particular form of the art of origami, where a number of individual units are combined to create three dimensional figures. The units are all identical, folded from sheets of paper into interlocking pieces. These pieces are then folded into one another, gradually piecing together a volumetric form.
The most exciting aspect of unit origami is the assembly of complex polyhedra. Such exotic figures as the icosahedron, tetragonal anti-prism, and stellated cubeoctahedron can all be pieced together from square pieces of paper.
Many of the basic units are easy to learn. And, because you have to make the same piece many times (the more complex shapes take several dozen units), you end up learning the folds pretty well.
Unit origami is also sometimes called modular origami.