Unlike regular origami, where a figure or sculpture such as a crane, frog, or airplane is made from a single sheet of paper without glue or cutting, modular origami figures are built up from multiple individually folded pieces, usually identical.
Modular origami is often very geometric.
Valerie Vann offers her own designs and developments, examples of other significant modular origami works, and favorite models by other modular origami creators. The Happy 8 puzzle. The Blithe 12 puzzle. Working through this caused fierce arguments between some students. Only when you start using explicit language does the difficulty crop up.