An index is understood to be the system used mainly but not exclusively in books with the aim of organizing and ordering the material that will be found throughout it. The index is a classified and more or less accessible presentation that seeks to allow the reader to find the most useful sections, as well as to establish a linear order necessary to carry out the reading. Traditionally, the index is built through titles, subtitles or numbers that serve as classifying elements of the information.
There are a number of ways to index a book, and an index can appear both at the beginning and at the end of the book, depending on the type of publication, the place of printing, and other details. On the other hand, while the best known index is the Table of Contents or the one that organizes the items in a linear way as they have been developed in the text, there is also the name index that presents the terms, characters or concepts found throughout the text with the page numbers in which they appear. Both types of indexes are necessary and useful for different types of searches.
The index should always serve so that the reader knows not only the contents that have been included in the work, but also how these have been classified and organized, as well as allowing to establish cross references and groups of concepts according to the specific need of each reader.
The term index is also used for other spaces beyond text publications. Always keep a goal of classification, organization and order of different types of content. For example, the index of a library will allow to know, find and use the books present in the different spaces in a fast, efficient and logical way. A statistical index is for example a classification of numbers and data that have been properly organized in such a way as to be useful for those who need to study them.