The concept of an illocutionary act is central to the concept of a speech act. Although there are numerous opinions regarding how to define 'illocutionary acts', there are some kinds of acts which are widely accepted as illocutionary, as for example promising or commanding - wikipedia 
Following the usage of, for example, John R. Searle, "speech act" is often meant to refer just to the same thing as the term illocutionary act, which John L. Austin had originally introduced in ''How to Do Things with Words'' (published posthumously in 1962). Searle's work on speech acts is also commonly understood to refine Austin's conception. However, some philosophers have pointed out a significant difference between the two conceptions: whereas Austin emphasized the conventional interpretation of speech acts, Searle emphasized a psychological interpretation (based on beliefs, intentions, etc.).
According to Austin's preliminary informal description, the idea of an "illocutionary act" can be captured by emphasizing that "by saying something, we ''do'' something", as when someone issues an order to someone to go by saying "Go!", or when a minister joins two people in marriage saying, "I now pronounce you husband and wife." (Austin would eventually define the "illocutionary act" in a more exact manner.)
An interesting type of illocutionary speech act is that performed in the utterance of what Austin calls performative utterances, typical instances of which are "I nominate John to be President", "I sentence you to ten years' imprisonment", or "I promise to pay you back." In these typical, rather explicit cases of performative sentences, the action that the sentence describes (nominating, sentencing, promising) is performed by the utterance of the sentence itself.
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