Synalepha

A synalepha or synaloepha 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 includes coalescence of vowels within a word. Similarly, synalepha most often refers to elision, but it can also refer to coalescence by other metaplasms: synizesis, synaeresis or crasis.

Examples

Spanish, Portuguese and Italian use synalepha, which is important in counting syllables in poetry. An example is in this hendecasyllable by Garcilaso de la Vega:
The words que and al form one syllable in counting them because of synalepha. The same thing happens with -ro and os- and so the line has eleven syllables :