aphesis

from WordNet (r) 3.0 (2006)

aphesis
    n 1: the gradual disappearance of an initial (usually
         unstressed) vowel or syllable as in `squire' for `esquire'

from The Collaborative International Dictionary of English v.0.48

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.
   [1913 Webster]