STL (STereoLithography) is a widely used 3D model file format that represents an object's surface as a mesh of triangular facets, serving as the standard input a slicer converts for 3D printing.
The STL format, originally created by 3D Systems for its early stereolithography machines, describes only the exterior surface geometry of a solid object. It does so by approximating that surface with many connected triangles, each defined by three vertices with x, y, and z coordinates plus a normal vector indicating the facet's outward-facing direction. Curved surfaces are approximated rather than exact, so a finer mesh with more triangles produces a closer approximation at the cost of a larger file.
STL exists in two encodings: a human-readable ASCII form and a more compact binary form, the latter far more common in practice because it keeps file sizes manageable. The format deliberately stores nothing but geometry — omitting color, texture, material, units, scale, and CAD feature history — which keeps it simple and near-universally supported across modeling and manufacturing software.
In the 📝3D printing workflow, an STL file is exported from CAD or modeling software and imported into a slicer, which reads the triangle mesh and generates the layer-by-layer machine instructions the printer executes. Its simplicity made STL the long-standing de facto interchange standard, though newer formats such as 3MF and OBJ address its limitations by carrying color, materials, and more precise geometry.