More complex indexes use tree-structured hierarchy like the above to show hierarchical relationships between topics.  In addition to regular "stand-alone" index entries that contain the entire topic as well as page number, you will often see general nouns listed (without page numbers) that have indented subentries that are more specific.  Such structures can be 2 levels, and sometimes even 3 levels deep.

 

An example you might see in the user's guide for a word processor:

 

 

Here, "editor window" is the general term.  Note that it is an index entry of its own, but does not have a specific page number associated with it.  It is there to provide context for the subentries.  Note that the subentries are listed in alphabetical order, and that they provide the actual page references.

 

For the above example, we say that the "editor window" entry is at "level 1" or is a "level-1 index entry", and its subentries are at "level 2" or are "level-2 index entries".  If there is a third level, these will be subentries for level-2 entries, which will provide context for the level-3 entries.

 

It is technically possible to have more than 3 levels, but for practical purposes, they are almost never seen in professional indexes.