An index is a list of anchors, links, or nodes ordered by letter, topic, author, subject, etc. This definition includes those tools common on paper: alphabetic indexes, tables of contents, glossaries, and others. Indexes provide a linear ordering of texts which have a more complex structure. This tool has the disadvantage of having to be created by the author without knowledge of how the reader may want to use the hypertext. The very existence of an index implies one particular means of accessing the text.
For an example of how this device can be used as a narrative in its own right, see JG Ballard's "The Index."