An ELT Glossary : Illocutionary Act / Illocutionary force / Function

Definition and examples :  Linguistics uses the terms "illocutionary act and "illocutionary force" with the same meaning as language teaching uses the term "function" (which has a different meaning in linguistics). The illocutionary force, or function, of an utterance is the action it is used to perform - eg the utterance "Why don't we go to the cinema?" has the function / carries the illocutionary force  of  making a suggestion; "Can you help me with this?" has the function / illocutionary force of making a request.

Further Information : Illocutionary force is dependent on context. In the dialogue ...

A : Why don't we get Sally a book for her birthday?    /    B : She hates reading.

B's utterance has the illocutionary force of rejecting a suggestion. However, the same utterance in a different context might have a completely different function - here, for example, it might constitute criticism :

A : She's never going to pass her exams. She hates reading and won't study.

Related Terms : Notion Functional-Notional Syllabus

Further reading

Jones, R.H.  Discourse analysis, Routledge