a·phaer·e·sis or a·pher·e·sis 
(ə-fĕrĭ-sĭs)
(ə-fĕrĭ-sĭs)n. pl. a·phaer·e·ses (-sēz′) or a·pher·e·ses
The loss of one or more sounds from the beginning of a word, as in copter from helicopter.
[Late Latin, from Greek aphairesis, from aphairein, to take away : apo-, apo- + hairein, to take.]
aph′ae·retic (ăf′ə-rĕtĭk) adj.