Apocope

In phonology, apocope ( ) is the omission (elision) or loss of a sound or sounds at the end of a word. While it most commonly refers to the loss of a final vowel, it can also describe the deletion of final consonants or even entire syllables.

For instance, in much spoken English, the t in the word don't is lost in the phrase I don't know, leading to the written representation I dunno.

The resulting word form after apocope has occurred is called an apocopation.