Info-index is an interactive autoloaded compiled Lisp function in `info.el'.

(Info-index TOPIC)

Look up a string TOPIC in the index for this manual and go to that entry.
If there are no exact matches to the specified topic, this chooses
the first match which is a case-insensitive substring of a topic.
Use the , command to see the other matches.
Give an empty topic name to go to the Index node itself.