DLL 
dynamic link library 
A program module that contains functions and data, intended to be shared by Microsoft Windows applications and/or the Windows operating system. DLLs are loaded into memory only when needed by another Windows program. And if multiple programs require the services of the same DLL, only one copy is loaded, conserving memory. At least, that was the original idea. Users of older versions of Windows often experienced what was known as "DLL hell," where a DLL installed by a new application would conflict with an existing DLL vital to the functioning of another application. Such conflicts were largely eliminated in Windows 2000 and Windows XP, which tend to load multiple DLL copies into memory rather than forcing applications to share one DLL.