Source : Webster's Revised Unabridged Dictionary (1913)

Aphesis \Aph"e*sis\, n. [Gr. ? a letting go; ? + ? to let go.]
   The loss of a short unaccented vowel at the beginning of a
   word; -- the result of a phonetic process; as, squire for
   esquire. --New Eng. Dict.

Source : WordNet®

aphesis
     n : the gradual disappearance of an initial (usually unstressed)
         vowel or syllable as in `squire' for `esquire'