In linguistics, coreference, sometimes written co-reference, occurs when two or more expressions in a text refer to the same person or thing; they have the same referent, e.g. Bill said he would come; the proper noun Bill and the pronoun he refer to the same person, namely to Bill. Coreference is the main concept underlying binding phenomena in the field of syntax. The theory of binding explores the syntactic relationship that exists between coreferential expressions in sentences and texts. When two expressions are coreferential, the one is usually a full form (the antecedent) and the other is an abbreviated form (a proform or anaphor). Linguists use indices to show coreference, as with the i index in the example Billi said hei would come. The two expressions with the same reference are coindexed, hence in this example Bill and he are coindexed, indicating that they should be interpreted as coreferential.
When exploring coreference, there are numerous distinctions that can be made, e.g. anaphora, cataphora, split antecedents, coreferring noun phrases, etc. When dealing with proforms (pronouns, pro-verbs, pro-adjectives, etc.), one distinguishes between anaphora and cataphora. When the proform follows the expression to which it refers, anaphora is present (the proform is an anaphor), and when it precedes the expression to which it refers, cataphora is present (the proform is a cataphor). These notions are illustrated as follows:
Semanticists and logicians sometimes draw a distinction between coreference and what is known as a bound variable. An instance of a bound variable can look like coreference, but from a technical standpoint, one can argue that it actually is not. Bound variables occur when the antecedent to the proform is an indefinite quantified expression, e.g.
Quantified expressions such as every student and no student are, from a technical standpoint, not referential. The subjects every student and no student are grammatically singular, but they do not pick out single referents in the discourse world. Thus since the antecedents to the possessive adjective his is not referential, one also cannot say that his is referential. Instead, one says it is a variable that is bound by its antecedent. Its reference varies based upon which of the students in the discourse world is thought of. If Jack, John, and Jerry are the three students in the discourse world, then the meaning of his varies based upon whether Jack, John, or Jerry is the focus of the minds eye. The existence of bound variables is perhaps more apparent with the following example:
This sentence is ambiguous. It can mean that Jack likes his grade, but everyone else dislikes Jack's grade, or more likely, it means that Jack likes his grade, but John dislikes his (John's) grade, and Jerry dislikes his (Jerry's) grade. The second, more natural reading is the bound variable reading. While the distinction between coreference and bound variables may be real, coindexation can be construed as accommodating both. That is, when two or more expressions are coindexed, it indicates that one is dealing with coreference or a bound variable.
In computational linguistics, coreference resolution is a well-studied problem in discourse. To derive the correct interpretation of a text, or even to estimate the relative importance of various mentioned subjects, pronouns and other referring expressions must be connected to the right individuals. Algorithms intended to resolve coreferences commonly look first for the nearest preceding individual that is compatible with the referring expression. For example, she might attach to a preceding expression such as the woman or Anne, but not to Bill. Pronouns such as himself have much stricter constraints. Algorithms for resolving coreference tend to have accuracy in the 75% range. As with many linguistic tasks, there is a tradeoff between precision and recall.
A classic problem for coreference resolution in English is the pronoun it, which has many uses. It can refer much like he and she, except that it generally refers to inanimate objects (the rules are actually more complex: animals may be any of it, he, or she; ships are traditionally she; hurricanes are usually it despite having gendered names). It can also refer to abstractions rather than beings: "He was paid minimum wage, but didn't seem to mind it." Finally, it also has pleonastic uses, which do not refer in anything specific:
Pleonastic uses are not considered referential, and so are not part of coreference.