Narrowly defined, an elision occurs when a letter or syllable is omitted from the end of one word or the beginning of the next word in a phrase, such as "he's" for "he is." More broadly, however, an elision may be the omission or slurring of a letter or syllable anywhere in a word, as it is spoken. Not bad.

I was hoping for a more derogatory term, but I can live with elision.