A cross-reference (noun) is an instance within a document which refers to related or synonymous information elsewhere, usually within the same work. To cross-reference or to cross-refer (verb) is to make such connections. The term "cross-reference" is often abbreviated as x-ref, xref, or, in computer science, XR. Cross-referencing is usually employed to either verify claims made by an author or to link to another piece of work that is of related interest. See, for example, the Linux cross-reference tool, LXR for init/main.c. In an index, a cross reference is often denoted by See Also. For example, under the term Albert Einstein in the index of a book about Nobel Laureates, there may be the cross-reference See Also: Einstein, Albert.
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Cross-referencing in hypertext (XR) is maintained internally or externally to a document with either in-context (XRIC) or out-of-context (XROC) cross-referencing. These are analogous to KWIC and KWOC, which were very early computer applications inherited from the centuries-old idea of concordance.
Traditionally, reference numbers and footnote marks are examples of in-context cross-referencing, whereas the index and the reference list at the end of texts are examples of out-of-context cross-referencing. Out-of-context cross-referencing relies on the traditional, manually-produced indexes using subject or citation. This remained the mainstream text retrieval system until the advent of CD-ROM in 1985, since which the digital text, the hypertext, and eventually the World Wide Web and search engines, provided systems for XRIC.
Soon after the advent of the Web, there was a rumor that XRIC features of the Web were better than the Gopher's XROC system. Charles Goldfarb, one of the founding pioneers in SGML, satirically compared the antagonism between XROC and XRIC paradigms to a religious war, which would be moderately called the cross-reference war. While the Web surpassed Gopher, in that XRIC is better than XROC in hypertext, both are as complementary as the two sides of the coin. Unfortunately, however, the schism between both text retrieval paradigms appears reflected on ACM/SIGIR and ACM/SIGWEB much overlapping each other.
The narrow or common sense of hypertext implies XRIC, while the wide or true sense includes XROC as well. From the text retrieval point of view, hypertext as a new retrieval paradigm, objecting to XROC or subjecting itself mainly to XRIC, sounds like a self-defeating misnomer, because text retrieval and cross-reference well comprise both XROC and XRIC in themselves. Ironically, hypertext was coined by Ted Nelson who used to object to the wide spectrum of text retrieval or cross-reference and subject it mainly to the narrow idea of transclusion, or simply quotation, aiming for text patchwork rather than retrieval.
A table can have an xref as prefix to indicate it is a cross-reference table that joins two tables together via primary key.
In printed and online dictionaries cross-references are important for several reasons. According to Nielsen (1999) they form a network structure of relations existing between different parts of data, dictionary-internal as well as dictionary external. The abstract mediostructure consists of all the possible sets of cross-referential relations. The actual realisation of these referential networks may be function-related, i.e. support a dictionary function such as translation. A distinction can be made between use-related and function-related cross-references. It is also possible to show hierarchical relationships (genus/species relation) between terms as well as sequential relations by using cross-references. The important point is that compilers of dictionaries need to take a broad approach to cross-references in dictionaries as they are directly linked to other structures in dictionaries.