Full Idea
Indexicals are expressions whose references depend on the circumstances of utterance, such as 'here', 'now', 'last month' 'I', 'you'. It was introduced by Peirce; Reichenbach called them 'token-reflexive', Russell 'ego-centric particulars'.
Gist of Idea
The references of indexicals ('there', 'now', 'I') depend on the circumstances of utterance
Source
Thomas Mautner (Penguin Dictionary of Philosophy [1996], p.272)
Book Reference
Mautner,Thomas: 'Dictionary of Philosophy' [Penguin 1997], p.272
A Reaction
Peirce's terminology seems best. They obviously create great problems for any theory of reference which is rather theoretical and linguistic, such as by the use of descriptions. You can't understand 'Look at that!' without practical awareness.