Synaeresis
Merging two contiguous vowels into one syllable: "lion" as one beat — diaeresis's inverse.
Synaeresis pronounces two contiguous vowels as a single syllable: French "lion," "hier," "ruine" counted as one beat. It's modern spoken language's spontaneous treatment — and using it in verse, where tradition would expect diaeresis, gives the poem a more oral, faster, more contemporary gait.
The diaeresis/synaeresis pair is one of French syllable-counting's main fine dials: many words change measure with the choice. Classical verse decides by rule; modern verse decides by ear — what matters is consistency: the same word shouldn't count two syllables in line 3 and three in line 9 without a perceptible reason.
Example
"Heaven" as one beat or two: the line decides.