Generating an index - The Org Manual

13.1.8 Generating an index

Org mode can generate an index across the files of a publishing project.

:makeindex When non-nil, generate in index in the file theindex.org and publish it as theindex.html.

The file will be created when first publishing a project with the :makeindex set. The file only contains a statement #+INCLUDE: "theindex.inc". You can then build around this include statement by adding a title, style information, etc.