In the philosophy of language and speech acts theory, performative utterances are sentences which not only describe a given reality, but also change the social reality they are describing.
In a 1955 lecture series, later published as How to Do Things with Words, J. L. Austin argued against a positivist philosophical claim that the utterances always "describe" or "constate" something and are thus always true or false. After mentioning several examples of sentences which are not so used, and not truth-evaluable (among them nonsensical sentences, interrogatives, directives and "ethical" propositions), he introduces "performative" sentences or illocutionary act as another instance.
In order to define performatives, Austin refers to those sentences which conform to the old prejudice in that they are used to describe or constate something, and which thus are true or false; and he calls such sentences "constatives". In contrast to them, Austin defines "performatives" as follows:
Performative utterances are not true or false, that is, not truth-evaluable; instead when something is wrong with them then they are "unhappy", while if nothing is wrong they are "happy".
The uttering of a performative is, or is part of, the doing of a certain kind of action (Austin later deals with them under the name illocutionary acts), the performance of which, again, would not normally be described as simply "saying" or "describing" something (cf. Austin 1962, 5).
The initial examples of performative sentences Austin gives are these:
"I do (sc. take this woman to be my lawful wedded wife)" – as uttered in the course of a marriage ceremony.
"I name this ship the 'Queen Elizabeth'"
"I give and bequeath my watch to my brother" – as occurring in a will
"I bet you sixpence it will rain tomorrow" (Austin 1962, 5)
As Austin later notices himself, these examples belong (more or less strikingly) to what Austin calls, explicit performatives; to utter an "explicit" performative sentence is to make explicit what act one is performing.