The illocutionary force of an utterance is the speaker's intention in producing that utterance. An illocutionary act is an instance of a culturally-defined speech act type, characterised by a particular illocutionary force; for example, promising, advising, warning, ..
Thus, if a speaker asks How's that salad doing? Is it ready yet?" as a way of ("politely") enquiring about the salad, his/her intent may be in fact to make the waiter bring the salad. Thus the illocutionary force of the utterance is not an inquiry about the progress of salad construction, but a demand that the salad be brought.
Illocutionary force is the combination of
See also identity of illocutionary forces
Sources Crystal 1980:152, Searle and Vanderveken 1985:1,7-9,20-21For more linguistic definitions, go to linguistic glossary.