A concordance is a comprehensive index of the words used in a text or a body of texts. Ordinarily it will not only index but also cite all passages in which a given word occurs. It is therefore useful for locating a passage that you cannot otherwise find, for discovering similar passages, or for establishing the frequency with which particular words are used.
Concordances exist for the Bible, for major poets, and even for some prose works. Compendia of famous quotations, such as Bartlett's, also function as concordances. Concordances can be important research tools for assessing themes, recurrent literary ideas, devices and tropes, and so forth.
To find concordances, search QCAT using a keyword search, e.g.
concordances and shakespeare
For a more complete list of religious concordances, go to the Religious Studies - Concordances subject guide.
The web abounds with texts that may be used in a fashion similar to concordances; simply use keywords and phrases to search the electronic text. You must consider the scholarly integrity of online texts as carefully as you would consider the merits of using different editions of the same text. Consult your professor, and ask: is the source of the web text documented properly?
The following websites furnish examples of reliable editions of online texts with concordance-like features:
Locke's "An Essay Concerning Human Understanding," (Indexed by Tze-wan Kwan and Chong-fuk Lau, Chinese University of Hong Kong)
The William Blake Archive (Edited by Morris Eaves, University of Rochester; Robert Essick, University of California, Riverside; and Joseph Viscomi, University of North Carolina at Chapel Hill) allows searches of Blake's writings and images.