A citation or bibliographic citation is a reference to a book, article, web page, or other published item with sufficient details to uniquely identify the item. Unpublished writings or speech, such as personal communications, are also sometimes cited. Citations are provided in scholarly works, bibliographies and indexes
Bibliographic management and citation formatting are central to this.
See the Wikiepedia Citation article.
A citation index is an index of citations between publications, allowing the user to easily establish which documents cite which other documents.
See the Citation index article for detail
There are many style conventions. In educational technology, the most popular one is probably APA.
A reference manager is a tool to manage references.