A Dynamic Link Library (.dll) is a Relocatable Objects]] (i.e., shared program module) with ordered code, methods, functions, enums and structures that may be dynamically called by an executing program during run time. A DLL usually has a file extension ending in .dll. Other file extensions are .drv and .ocx.
DLLs were developed by Microsoft and work only with the Windows Operating System (OS).
Source: https://www.techopedia.com/definition/24835/dynamic-link-library-dll