A global array created by Tcl on startup, where keys hold proc names, and the value is a script indicating how to make the key-named proc available (typically [source] with full pathname of a Tcl file). Tcl initialises the array by sourcing [tclIndex] files. See the [library] manpage for more.

For finding the location of the source for [auto_load]ed commands, this array is very useful. E.g., for [parray] do
  set auto_index(parray)
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