What is the term given to a scenario where words are contarcted in a poem?

Syncope

Explanation:

The term syncope refers to a device of literature that implies the contraction or reduction of a word by the omission of syllables, sounds, or letters in between the word, for example, the expression "'tis" for the phrase "it is". Syncope can also imply suppressing unstressed syllables, vowels, or letters as well as consonants.