Turbo Pascal Unit
A Turbo Pascal Unit is a separately-compiled file generated using Turbo Pascal or Borland Pascal for the purpose of linking at build time into an executable file making use of multiple modules. TPU files were introduced in Turbo Pascal 4.0; earlier versions instead used chain files that were "chained to" at execution time.
Different versions of Turbo/Borland Pascal generated different versions of TPU files, so when you upgraded to a new Pascal version you would probably have to recompile your units. However, within the same Pascal version, you didn't need to recompile them unless you changed their source code, which could greatly reduce build time for a complicated project containing many units.
The source code to a unit contains the statement
unit unitname, while other units (and main programs) that use it had
uses unitname. A unit could be used in many different programs.