LD_LIBRARY_PATH is an environmental variable, used by Unix like operating systems, to specify one or more directories in which the operating system should look for dynamic libraries. There are a set of directories in which the operating system will look by default. However, when a package, such as Tcl, installs its dynamic libraries into a directory other than one of those default ones, the user (or developer if they are kind) will need to specify the correct directory before the operating system is able to locate the needed libraries.