Synalepha

A synalepha or synaloepha /ˌsɪnəˈliːfə/ is the merging of two syllables into one, especially when it causes two words to be pronounced as one.

The original meaning in Ancient Greek is more general than modern usage, and also includes coalescence of vowels within a word. Similarly, synalepha most often refers to elision (as in English contraction), but it can also refer to coalescence by other metaplasms: synizesis, synaeresis, or crasis.