Definitions for "Speech act"
a declarative sentence can be used to perform a number of different speech acts. In uttering It's cold in here a speaker may perform an act of requesting the hearer to close the window or turn up the heating.
a communicative activity, defined with reference to what is said by the speaker, the intentions of the speaker while speaking, and the effects achieved on the listener.
Technical term in philosophy of language to refer to the different functions of language, e.g., locutionary, illocutionary, perlocutionary, constative, performative or, more simply, communicating, promising, commanding, naming, etc.