An index is a finding tool that enables you to look up a topic and quickly learn where information on that topic is located within a specific work.
For example, many print books have an index, usually at the back. The index is an alphabetical list of the topics in the book. Under each topic, page numbers are given that show where information on that topic can be located.
Specialist subject indexes are useful when you know the subject you are interested in and want to find relevant journal articles and book reviews on your topic. Many subject indexes are now available online, for example, the Cumulative Index to Nursing and Allied Health (CINAHL) indexes articles from hundreds of journals. You can do a subject or keyword search on Heart Surgery for example and get a list of citations to articles on that topic.