Mostly Harmless

From Unreal Wiki, The Unreal Engine Documentation Site

Object >> Primitive (native-only)

Package: 

Engine

Direct subclasses:

Mesh, StaticMesh

This class in other games:

RTNP, U1, U2, UT, UT2003, UT2004

The base class for 3D primitives, like vertex meshes, static meshes or terrain.