GLOSSARY
STL File
STL (stereolithography) is a 3D file format that stores a model as a list of triangles. It is the lingua franca of 3D printing — every slicer reads it.
Definition
An STL file describes a 3D shape as a flat list of triangles. Each triangle has three vertex coordinates and a normal vector that points to the outside of the surface. There is no color, no texture, no units, no hierarchy — just geometry.
The format was created by 3D Systems in 1987 for stereolithography machines, and the name stuck even as it became the default for FDM, MSLA, SLS, and almost every other process. It comes in two encodings: ASCII (human-readable, large) and binary (smaller, what everyone actually uses).
Why it matters
STL is what your slicer expects. When you generate a model in Automatic3D, export from Blender or Fusion 360, or download a print from Thingiverse, you are working with STL. Cura, PrusaSlicer, Bambu Studio, OrcaSlicer — all of them ingest STL by default.
Because the format is so simple, it is also forgiving. Even a roughly generated mesh will load. The trade-off is that nothing in an STL file tells you whether the geometry is actually printable. That part — manifold, watertight, correctly oriented normals — is on you.
Common confusion
STL files have no units. A "1.0" in the file might be a millimeter, an inch, or a meter — the slicer assumes millimeters unless told otherwise. This is why a model can come in absurdly small or large when you import it.
STL also cannot store color or material. If you need painted multi- color prints, you want 3MF or OBJ. If you need parametric edits, you want STEP.