from The American Heritage® Dictionary of the English Language, 4th Edition
- n. The blending into one syllable of two successive vowels of adjacent syllables, especially to fit a poetic meter; for example, th' elite for the elite.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. The suppression of a vowel at the end of word when it is followed by another word beginning with a vowel.
- n. The melding into a single syllable of two vowels from two different syllables.
from the GNU version of the Collaborative International Dictionary of English
- n. A contraction of syllables by suppressing some vowel or diphthong at the end of a word, before another vowel or diphthong.
The final vowel or diphthong of one word and the initial vowel or diphthong of an immediately following word in the same line usually combine to form one syllable (this is called "synalepha")  as in:
In each case allowing for synalepha, the thirteenth syllable is stressed.
Cf. also _rendíos_, etc., where the _o_ of _os_ combines with the _í_ by synalepha.
The forced synalepha of = yo haga = is discordant and incorrect.
And synalepha is the rule, if stress on the initial syllable is weak:
In singing Spanish verses two facts are of especial interest: that, where the rules of prosody require synalepha, hiatus sometimes occurs (especially in opera), thus:
But synalepha is possible (especially of _de o-_):
In modern Spanish, _h_, being silent, has no effect, but in older Spanish, _h_ for Latin _f_, being then pronounced, prevented synalepha, as in:
But synalepha may occur in combinations of vowels in which syneresis would be impossible.
The only places where there is no synalepha are in the third line between ‘no’ and ‘amo’, and in the last line, between ’solamente’ and ‘entre’.