Dynamic Link Library, a library of executable functions or data that can be used by a Windows application. Typically, a DLL provides one or more particular functions and a program accesses the functions by creating either a static or dynamic link to the DLL. A static link remains constant during program execution while a dynamic link is created by the program as needed. DLLs can also contain just data. DLL files usually end with the extension .dll,.exe., drv, or .fon.

A DLL can be used by several applications at the same time. Some DLLs are provided with the Windows operating system and available for any Windows application. Other DLLs are written for a particular application and are loaded with the application.

Delay Locked Loop, Delay-Locked Loop (DLL) supports high-bandwidth data rates between devices. These DLLs are circuits that provide zero propagation delay, low-clock skew between output clock signals throughout a device, and advanced clock domain control. These dedicated DLLs can be used to implement several circuits that improve and simplify system level design.
These are just a few samples of what DLL stands for. To learn more, Google DLL and you will see that DLL stands for hundreds of different meanings.