From the Greek synaleiphein, meaning ‘to smear (or melt) together’.
Synaloepha is the contraction of two syllables into one by omission
of vowel. They are often contractions (by which I mean that they are written with an apostrophe to indicate the deletion), but not always. It can also be used more generally to mean any contraction of two syllables into one.
A simple example would be it is into it’s. Synaloepha is a form of metaplasm, implying that the contraction took place for artistic reasons, for example, to fit a poetic meter better.
Also spelt synalepha, synaloephe, synolepha, and episynaloepha.
My senses lead thro' flow'ry paths of joy;
But let the sacred Genius of the night
Such mystic visions send, as Spenser saw,
When thro' bewild'ring Fancy's magic maze,
To the fell house of Busyrane, he led
Th' unshaken Britomart; or Milton knew,
When in abstracted thought he first conceiv'd
All heav'n in tumult, and the Seraphim
Come tow'ring, arm'd in adamant and gold.
--The Pleasures of Melancholy by Thomas Warton