Mesh

In ROBLOX, a mesh is a 3D object, such as a hat (catalog item), gear (catalog item) or Part. Every hat, gear, and package, except the Mysterious Box, is made up of a mesh(es).

There are SpecialMeshes, which contain BlockMeshes and CylinderMeshes.

User Meshes
From 2008 to 2010, players could upload their own custom meshes via a simple exploit made public by Gamer3D. However, the user-made meshes could only be seen by placing the mesh in a game while in Roblox Studio. In early 2011, this feature was removed due to some "inappropriate content". Despite this, however, many users have formed groups demanding meshes to be returned as an official feature.