(STereoLithography) A 3D printing file format created by 3D Systems for its stereolithography system. Also supported by many numerical control, rapid prototyping and rapid manufacturing machines, STL provides the surface geometry of the item in triangles. STL comes in both ASCII and binary versions, the latter a more compact file because the triangles are expressed in floating point. In addition, some binary STL variations include color attributes. See 3D printing.