Polygon soup
A pile of triangles with no information about how they connect — no shared edges, no promise that the surface is closed.
A well-built mesh knows which triangles are neighbours, which edges they share and which side is inside. A polygon soup knows none of that: it is just a list of vertices in threes, usually with overlaps, gaps, duplicates and faces poking through each other. It is exactly what you get when a level is assembled from modular pieces shoved together in an editor.
Rendering rarely cares — the GPU draws one triangle at a time and never asks about neighbours. Everything else does. Classic BSP compilers demanded a sealed world and refused with the dreaded "leak" error. Tools like Recast were therefore made deliberately soup-tolerant: the geometry is voxelised first, at which point topology stops mattering. Mesh colliders cope with soup too, but thin single-sided faces are where players end up falling out of the world.