Elision: an unstressed syllable (see prosody) that does not count in the metrical tally, usually having been omitted in order to make the line conform to a metrical pattern. A phrase such as “the imperial towers” has seven syllables in a careful enunciation, with seven vowels (numbered below):
The - im - pe- ri- al - tow- ers
1 2 3 4 5 6 7
However, in Pope's line from The Rape of the Lock only four of those vowels count in the metre; number 1 is elided before the following vowel (synaloepha), number 4 turns to a semivowel ( as in yet) before the following vowel …
Groves, Peter Lewis. "Elision". The Literary Encyclopedia. First published 01 November 2001; last revised 11 January 2005.
[http://www.litencyc.com/php/stopics.php?rec=true&UID=322, accessed 16 January 2017.]