In general, a reference is a relation between objects in which one object designates by linking to another object. Such relations as these may occur in a variety of domains, including logic, computer science, time, art and scholarship.

Although the objects which the term reference applies may be of a varying character ranging from concrete examples such as reference work which includes pointers or symbols.

The nature of reference as a role in language and thought has been around since the 19th Century. During this time, applying itself as an important topic of discussion. An object which is referred to as a reference (where the reference leads) is called a referent.

In a library, the word reference may refer to a dictionary, an encyclopedia or other reference work, that contains many brief articles that cover a broad scope of knowledge in one book, or a set of books.

However, the word reference is also used to mean a book that cannot be taken from the room, or from the building. Many of the books in the reference department of a library are reference works, but some are books that are simply too large or valuable to loan out.

Conversely, selected reference works may be shelved with other circulating books, and may be loaned out.

References to many types of printed matter may come in an electronic or machine-readable form. For books, there exists the ISBN and for journal articles, the Digital object identifier (DOI) is gaining relevance.

Information on the internet may be referred to by a Uniform Resource Identifier (URI).

Librarians also conduct reference interviews at library reference desks, to help people find the information they seek. Help may also be available outside the library though virtual reference and digital reference services.