The .STL extension refers to files of the type, Stereolithography files. It is a native file format for CAD software that are based on stereolithography. There are many community developed and supported packages that work with the STL file and language for CAD.

The STL file is mostly used for rapid prototyping and contains usually 3D modeling data. Some of the various versions of STL are ASCII STL, Binary STL and the native STL. The 3D printers are based on the principle of stereolithography and hence the use of STL is widely seen in the industry.