Many define the term with reference to examples, saying such things as that any speech act like stating, asking, commanding, promising, and so on is an illocutionary act; they then often fail to give any sense of the expression illocutionary act capable of making clear what being an illocutionary act essentially consists in.
It is also often emphasised that Austin introduced the illocutionary act by means of a contrast with other kinds of acts: 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.
Still another conception of an illocutionary act goes back to Schiffer's famous book 'Meaning' (1972, 103), in which the illocutionary act is represented as just the act of meaning something.
According to the conception Bach and Harnish adopt in 'Linguistic Communication and Speech Acts' (1979), an illocutionary act is an attempt to communicate, which they again analyse as the expressing of an attitude.
According to a widespread opinion, an adequate and useful account of "illocutionary acts" has been provided by John R. Searle (e.g., 1969, 1979). In recent years, however, it has repeatedly been doubted whether Searle's account is well-founded. A wide-ranging critique is in F C Doerge Illocutionary Acts Whole collections of articles examining Searle's account are: Burkhardt 1990 and Lepore / van Gulick 1991.
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 been 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.
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.
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.