3-manifold

A 3-manifold is a shape that, if you look at any small enough piece of it, appears like regular 3D space around you—similar to how a surface of the Earth looks flat locally, even though globally it's spherical. In essence, it's a space where every tiny neighborhood resembles the usual three-dimensional universe we live in. These objects can be very complex and are studied in topology to understand their global structure, classification, and possible forms. Think of them as the higher-dimensional analogs of surfaces (2-manifolds), extended into three dimensions.