auto_load, a command in the Tcl library, attempts to load the definition for a command.
auto_load, looks in the directories specified by $auto_path for files named tclIndex. These are "Tcl autoload index files", and are formatted in two versions; version 2.0 contains commands set auto_index(::namespace::function_name) $cmd where $cmd typically sources a file that defines the command ::namespace::function_name