hyphaeresis
noun
hy·phaer·e·sis hī-ˈfer-ə-səs 
especially British -ˈfir-
: the omission of a sound, letter, or syllable from the body of a word
Word History
Etymology
borrowed from Greek hyphaíresis "taking from below, subtraction, omission of a letter," from hyphaireîn "to draw from under, pilfer, subtract" (from hyp- hyp- + haireîn "to take, grasp," of obscure origin) + -sis -sis
First Known Use
1884, in the meaning defined above