Manifold: A manifold mesh is one where every edge is shared by exactly two faces, so the surface could exist as a real solid object, which is what slicers and quoting software need.
The word comes from geometry, but the rule in practice is simple. Look at any edge of the mesh: exactly two triangles should meet there, one on each side. Where that is true everywhere, the surface is manifold and can describe a real, physical object.
Non-manifold geometry breaks that rule in a few common ways:
- an edge shared by three or more faces, such as two boxes touching along one edge;
- an open edge with only one face, which is a hole (the mesh is not watertight);
- two parts meeting at a single point;
- internal faces left inside the solid after a modelling operation.
Example: a designer places two cubes so they share one edge exactly. On screen it looks like one object. In the file, that edge belongs to four faces. A slicer cannot decide whether the space along the edge is inside or outside, so the print may have a gap or a blob there.
Manifold and watertight are often used as if they meant the same thing. Every manifold mesh with no boundary edges is watertight, but a watertight mesh can still be non-manifold if, for example, three faces share an edge.
Non-manifold meshes are a common reason a file fails analysis or prints badly. Good CAD exports rarely produce them; scans, game models and hand-edited meshes often do. Read design for 3D printing for more file checks.
Go deeper: Design for 3D printing: walls, overhangs, holes, clearances and orientation