DLL hell is the problem that occurs when an installation of a newer application might break other applications as newer DLLs are copied into the system the old application doesn't work.

DLL Hell refers to the set of problems caused when multiple applications attempt to share a common component like a dynamic-link library (DLL) or a Component Object Model (COM) class. In the most typical case, one application will install a new version of the shared component that is not backward compatible with the version already on the machine. Although the application that has just been installed works fine, existing applications that depended on a previous version of the shared component might no longer work.