Design Nomenclature / Form & Geometry
Manifold
Also called non-manifold, 2-manifold, manifold geometry
This entry has not been translated yet and is shown in English.
A surface on which every point has a neighbourhood that looks like a flat disc. For a mesh this reduces to two rules: every edge is shared by exactly two faces, or by one at an open boundary; and the faces around each vertex form a single fan. Three faces on an edge, or two cones meeting at a point, breaks it.
In practice
Slicers, booleans, and solvers refuse non-manifold input; renderers do not care at all. 'Non-manifold geometry' in an error dialog is nearly always a bowtie vertex or an internal wall left behind by a bad boolean or a face extruded in place.