DLL
Windows's shared library format — a .dll file is code loaded into a process at runtime.
Definition
A Dynamic-Link Library is a Windows shared library containing code or resources that multiple processes can load at runtime. The Linux equivalent is .so; macOS uses .dylib. DLL load failures (missing dependency, version mismatch, security policy block) are a common class of Windows install error, often surfaced as cryptic HRESULT codes.