Elision is a figure of Speech. It is one of a group which deal with the the contraction or shortening of words.
Elision is the omission of a letter or syllable to allow two sounds to run into one. (There is a verb 'to elide'.) Shakespeare not uncommonly elides the two words "in the" into one syllable, "i'thâ€™". A special form of ellipsis is AphÃ¦resis. Another special form is syncope or syncopation, which, in language study, means: "Contraction of a word by omission of one or more syllables or letters in the middle." (OED)