Design Nomenclature / Digital Model Making
STL
Also called stereolithography file, .stl, STL file
An unstructured list of triangles, each written as three vertices and a normal. That is all: no units, no scale, no colour, no materials, no assemblies, no curves, and no shared vertices — a point is repeated once per triangle touching it. Nothing in the format requires the triangles to form a closed solid.
In practice
Half of all 3D-printing problems are STL problems: wrong scale because there are no units, holes because nothing enforced manifoldness, faceted curves because the tessellation was fixed at export.
Not to be confused with
- OBJA plain-text mesh format carrying vertices, faces of any size, UVs, normals, and material references in a sidecar .mtl file. No animation, no rig, no scene hierarchy, no unit convention. Ubiquitous because it is trivial to parse and everything reads it — not because it is good at anything.
- STEPAn open ISO-standardized CAD exchange format carrying exact B-rep geometry — trimmed spline surfaces, topology, assemblies, units, and in AP242 tolerances and annotation. It has no feature history: you receive an exact solid you can measure and machine, not one you can rebuild by changing a dimension. IGES is its older, surfaces-only ancestor.
Origin
Named for stereolithography, the first commercial 3D-printing process (1987). It was a machine driver's input file, not an interchange format, and was never meant to still be in use forty years on.