Phatic communication (or communion) is communication which has a purely social function and no informative value. For example, if you meet someone in the street, they may say All right?, Hello, how's things? or Nice day today without needing (or actually wanting) an informative reply. The phrase serves only to greet and acknowledge the existence of the other person.
Phatic communication is therefore generally formulaic - a "truthful" reply is not needed, but simply a return acknowledgement of the other person's greeting. The person who says All right? or How are you? probably doesn't want to be told that you are going down with the 'flu, have a sore throat and the beginnings of a temperature, but just expects a reply like Great thanks.
As Boxer states : Communicatively competent members of speech communities recognize these formulae as greetings (i.e. interactional) rather than requests for detailed information (i.e. transactional). The extent to which the question "how are you?" is phatic is reflected in our propensity, as native speakers, to answer with a phatic "fine" even in contexts outside the social domain (e.g doctor's office visits). (2002:49)
The formulae used in phatic communication will differ from culture to culture. For example, in Chinese Have you eaten? is a purely phatic greeting, as is Are you going somewhere? in Korean.
The term phatic communion was first used by the anthropologist Malinowski in 1923.
References
Boxer, D. (2002) Applying Sociolinguistics: Domains and Face-to-Face Interaction.
John Benjamins Publishing Company
Malinowski, B. (1923) "The Problem of Meaning in Primitive Languages”, in Ogden C.K and Richards, I.A. (eds.), The Meaning of Meaning, London: Routledge
