In general, a reference is a relation between objects in which one object designates, or acts as a means by which to connect to or link to, another object. Such relations may occur in a variety of domains, including linguistics, logic, computer science, art, and scholarship. Thus, the objects to which the term reference applies may be of varying character ranging from concrete examples, such as a reference work in a library, as well as abstract objects, such as pointers in computer programming languages or symbols in language or mathematics. The nature of reference in its role in language and thought has been a prominent topic of discussion in philosophy since at least the 19th century and the logic work of J.S. Mill.

An object which is named by a reference, or to which the reference points, is called a referent.
The term reference is used with different specialized meanings in a variety of fields, as follows:

Semantics
In semantics, reference is generally construed as the relation between nouns or pronouns and objects that are named by them. Hence the word John refers to John. The word it refers to some previously specified object. The object referred to is called the referent of the word.[2] Sometimes the word-object relation is called denotation; the word denotes the object. The converse relation, the relation from object to word, is called exemplification; the object exemplifies what the word denotes. In syntactic analysis, if a word refers to a previous word, the previous word is called the antecedent.

Reference and meaning
Frege argued that reference cannot be treated as identical with meaning: "Hesperus" (an ancient Greek name for the evening star) and "Phosphorus" (an ancient Greek name for the morning star) both refer to Venus, but the astronomical fact that '"Hesperus" is "Phosphorus"' can still be informative, even if the 'meanings' of both "Hesperus" and "Phosphorus" are already known. This problem led Frege to distinguish between the sense and reference of a word.

Absent referent
Words can often be meaningful without having a referent. Fictional and mythological names such as "Bo-Peep" and "Hercules" illustrate this possibility.

For those who argue that one cannot directly experience the divine (e.g. God), the sign "God" can serve as an example of a reference with an absent referent. Additionally, certain sects of Judaism and other religions consider it sinful to write, discard, or deface the name of the divine. To avoid this problem, the signifier G-d is sometimes used, though this could be seen as a sign which refers to another sign with an absent referent.

In mathematics, the absent referent can be seen with the symbol for zero, "0" or the empty set, "{ }".

Linguistic sign
The semantic sign can be considered a subset of a more general concept, the linguistic sign, first elucidated by Ferdinand de Saussure‎. A sign contains two parts, the signified (a thought which represents an object), and the signifier (the sound or written word). Both have a referent (the actual physical object). The sign is a building block for texts that supplies sound and meaning. The smallest building block is called a morpheme and may be lexical (carry lexical/encyclopedic meaning, e.g. refer to real-life entities) - or grammatical, i.e. combine with lexical ones, denoting another kind of reference - to actors (agreement), to time (tempus) etc., depending on the part of speech and indeed on the language.