An ELT Glossary : Pragmatics
Pragmatics studies how language is used in context (including the relationship between the speakers) and how meaning is interpreted in specific situations. Widdowson (1978:29) gave the following example :
A : That's the phone.
B : I'm in the bath.
A : OK.
As Widdowson points out, the first utterance, though a declarative sentence, is clearly interpreted by B as being a request to answer the phone, and B's reply is interpreted by A as a refusal and excuse. Although, as Widdowson says, there are no explicit cohesive links between the utterances, recognising how the speakers interpret their illocutionary force enable us to supply the "missing" propositional links and recognise it as coherent discourse. :
A : That's the phone. (Can you answer it, please?)
B : (No I can't answer it because) I'm in the bath.
A : OK (I'll answer it).
This interpretation is made possible by the speakers' knowledge of the world : phones need to be answered when they ring, if you're in the bath it's difficult to get to the phone in time to answer it etc.
See also : Pragmatic competence Functions Adjacency pairs
Widdowson, H.G. (1978) Teaching English as Communication, Oxford University Press