Definition:

In pragmatics (and other branches of linguistics and philosophy), the features of a language that refer directly to the circumstances or context in which an utterance takes place.

An indexical expression (such as today, that, here, utterance, and you) is a word or phrase that is associated with different meanings (or referents) on different occasions.

In conversation, interpretation of indexical expressions may in part depend on a variety of paralinguistic and non-linguistic features, such as hand gestures and the shared experiences of the participants.

See also:

• Context Sensitivity
• Deictic Word
• Disambiguation
• Rhetorical Situation
• Semiotics
• Speech Act

Examples and Observations:

• "Among philosophers and linguists, the term indexicality typically is used to distinguish those classes of expressions, like this and that, here and now, I and you, whose meaning is conditional on the situation of their use, from those such as, for example, noun phrases that refer to a class of objects, whose meaning is claimed to be specifiable in objective, or context-free terms. But in an important sense, namely a communicative one, the significance of a linguistic expression is always contingent on the circumstances of its use. In this sense, deictic expressions, place and time adverbs, and pronouns are just particularly clear illustrations of a general fact about situated language."
  (Lucy A. Suchman, "What Is Human--Machine Interaction?" Cognition, Computing, and Cooperation, ed. by Scott P. Robertson, Wayne Zachary, and John B. Black. Ablex, 1990)
• "The success of a deictic act of reference to a given book by means of an indexical expression like This book, for instance, requires the presence of the book within the visual field shared by the interlocutors, just like its gestural indication. But indexical expressions are not necessarily put to deictic use. Definite noun phrases and third person pronouns allow for anaphoric and cataphoric use. During anaphoric indication, the expression remains the same, but the field undergoes a change. The expression does not typically refer to an individual physically given in the perceptual field, but necessarily refers to an entity previously or subsequently named within the same discourse or text: I'm reading a paper on cataphora. I find it (this paper) interesting."
  (Michele Prandi, The Building Blocks of Meaning: Ideas for a Philosophical Grammar. John Benjamins, 2004)