Illocutionary act is a term in linguistics introduced by the philosopher John L. Austin in his investigation of the various aspects of speech acts. We may sum up Austin's terminology with the following example. In uttering the locution "Is there any salt?" at the dinner table, one may thereby perform the illocutionary act of requesting salt, as well as the distinct locutionary act of uttering the interrogatory sentence about the presence of salt, and the further perlocutionary act of causing somebody to hand one the salt.
The notion of an illocutionary act is closely connected with Austin's doctrine of the so-called 'performative' and 'constative utterances': an utterance is "performative" just in case it is issued in the course of the "doing of an action" (1975, 5), by which, again, Austin means the performance of an illocutionary act (Austin 1975, 6 n2, 133). According to Austin's original exposition in How to Do Things With Words, an illocutionary act is an act (1) for the performance of which I must make it clear to some other person that the act is performed (Austin speaks of the 'securing of uptake'), and (2) the performance of which involves the production of what Austin calls 'conventional consequences' as, e.g., rights, commitments, or obligations (Austin 1975, 116f., 121, 139). Thus, for example, in order to make a promise I must make clear to my audience that the act I am performing is the making of a promise, and in the performance of the act I will be undertaking an obligation to do the promised thing: so promising is an illocutionary act in the present sense. Since Austin's death, the term has been defined differently by various authors.
Approaches to defining "illocutionary act" 
Many define the term "illocutionary act" with reference to examples, saying for example that any speech act (like stating, asking, commanding, promising, and so on) is an illocutionary act. This approach has generally failed to give any useful hints about what traits and elements make up an illocutionary act; that is, what defines such an act. It is also often emphasised that Austin introduced the illocutionary act by means of a contrast with other kinds of acts or aspects of acting: the illocutionary act, he says, is an act performed in saying something, as contrasted with a locutionary act, the act of saying something, and also contrasted with a perlocutionary act, an act performed by saying something. Austin, however, eventually abandoned the "in saying" / "by saying" test (1975, 123).
According to the conception adopted by Bach and Harnish in 'Linguistic Communication and Speech Acts' (1979), an illocutionary act is an attempt to communicate, which they analyse as the expression of an attitude. Another conception of the illocutionary act goes back to Schiffer's book 'Meaning' (1972, 103), in which the illocutionary act is represented as just the act of meaning something.
According to a widespread opinion, an adequate and useful account of "illocutionary acts" has been provided by John Searle (e.g., 1969, 1975, 1979). In recent years, however, it has been doubted whether Searle's account is well-founded. A wide ranging critique is in FC Doerge 2006. Collections of articles examining Searle's account are: Burkhardt 1990 and Lepore / van Gulick 1991.
Classes of illocutionary acts 
Searle (1975) set up the following classification of illocutionary speech acts:
- assertives = speech acts that commit a speaker to the truth of the expressed proposition
- directives = speech acts that are to cause the hearer to take a particular action, e.g. requests, commands and advice
- commissives = speech acts that commit a speaker to some future action, e.g. promises and oaths
- expressives = speech acts that express on the speaker's attitudes and emotions towards the proposition, e.g. congratulations, excuses and thanks
- declarations = speech acts that change the reality in accord with the proposition of the declaration, e.g. baptisms, pronouncing someone guilty or pronouncing someone husband and wife
Illocutionary force 
Several speech act theorists, including Austin himself, make use of the notion of an illocutionary force. In Austin's original account, the notion remains rather unclear. Some followers of Austin, such as David Holdcroft, view illocutionary force as the property of an utterance to be made with the intention to perform a certain illocutionary act -- rather than as the successful performance of the act (which is supposed to further require the appropriateness of certain circumstances). According to this conception, the utterance of "I bet you five pounds that it will rain" may well have an illocutionary force even if the addressee doesn't hear it. However, Bach and Harnish assume illocutionary force just in case this or that illocutionary act is actually (successfully) performed. According to this conception, the addressee must have heard and understood that the speaker intends to make a bet with them in order for the utterance to have 'illocutionary force'.
If we adopt the notion of illocutionary force as an aspect of meaning, then it appears that the (intended) 'force' of certain sentences, or utterances, is not quite obvious. If someone says, "It sure is cold in here", there are several different illocutionary acts that might be aimed at by the utterance. The utterer might intend to describe the room, in which case the illocutionary force would be that of 'describing'. But she might also intend to criticise someone who should have kept the room warm. Or it might be meant as a request to someone to close the window. These forces may be interrelated: it may be by way of stating that the temperature is too cold that one criticises someone else. Such a performance of an illocutionary act by means of the performance of another is referred to as an indirect speech act.
Illocutionary force indicating devices (IFIDs) 
Searle and Vanderveken (1985) often speak about what they call 'illocutionary force indicating devices' (IFIDs). These are supposed to be elements, or aspects of linguistic devices which indicate either (dependent on which conceptions of "illocutionary force" and "illocutionary act" are adopted) that the utterance is made with a certain illocutionary force, or else that it constitutes the performance of a certain illocutionary act. In English, for example, the interrogative mood is supposed to indicate that the utterance is (intended as) a question; the directive mood indicates that the utterance is (intended as) a directive illocutionary act (an order, a request, etc.); the words "I promise" are supposed to indicate that the utterance is (intended as) a promise. Possible IFIDs in English include: word order, stress, intonation contour, punctuation, the mood of the verb, and performative verbs.
Illocutionary negations 
Another notion Searle and Vanderveken use is that of an 'illocutionary negation'. The difference of such an 'illocutionary negation' to a 'propositional negation' can be explained by reference to the difference between "I do not promise to come" and "I promise not to come". The first is an illocutionary negation - the 'not' negates the promise. The second is a propositional negation. In the view of Searle and Vanderveken, illocutionary negations change the type of illocutionary act.
See also 
- Direction of fit
- J. L. Austin
- John Searle
- performative utterance
- perlocutionary act
- speech act
- Discussion of illocutionary acts in sec. 1 of Stanford Encycolopedia of Philosophy, "Assertion", http://plato.stanford.edu/entries/assertion/.
- Alston, William P.. Illocutionary Acts and Sentence Meaning. Ithaca: Cornell University Press. 2000
- Austin, John L.. How To Do Things with Words. Oxford: Oxford University Press. 1975 ISBN 0-19-281205-X
- Burkhardt, Armin (ed.). Speech Acts, Meaning and Intentions: Critical Approaches to the Philosophy of John R. Searle. Berlin / New York 1990 ISBN 0-89925-357-1
- Doerge, Friedrich Christoph. Illocutionary Acts - Austin's Account and What Searle Made Out of It. Tuebingen 2006. 
- Lepore, Ernest / van Gulick, Robert (eds). John Searle and his Critics. Oxford: Basil Blackwell 1991. ISBN 0-631-15636-4
- Searle, John R. Speech Acts. Cambridge University Press. 1969 ISBN 0-521-07184-4
- Searle, John R. “A Taxonomy of Illocutionary Acts”, in: Günderson, K. (ed.), Language, Mind, and Knowledge, Minneapolis, vol. 7. 1975
- Searle, John R. Expression and Meaning. Cambridge University Press. 1979 ISBN 0-521-22901-4
- Searle, John R. and Daniel Vanderveken. Foundations of Illocutionary Logic. Cambridge University Press. 1985. ISBN 0-521-26324-7