Elision

The use of a speech form that lacks a final or initial sound which a variant speech form has (such as 's instead of is in there's). Elision of sounds can ... be seen clearly in contracted forms like isn't (is not), I'll (I shall/will), who's (who is/has), they'd (they had, they should, or they would), haven't (have not) and so on. We see from these examples that vowels or/and consonants can be elided.

It is very important to note that sounds do not simply 'disappear' like a light being switched off. A transcription such as /æks/ for acts implies that the /t/ phoneme has dropped out altogether, but detailed examination of speech shows that such effects are more gradual: in slow speech the /t/ may be fully pronounced, with an audible transition from the preceding /k/ and to the following /s/, while in a more rapid style it may be articulated but not given any audible realization, and in very rapid speech it may be observable, if at all, only as a rather early movement of the tongue blade toward the /s/ position.

Sources:

Crowley, T. (1997) An Introduction to Historical Linguistics(3rd edition). Oxford University Press.

Reboul, O. (1991). Introduction à la rhétorique. Paris: Presses Universitaires de France.