The gradual disappearance of an initial (usually unstressed) vowel or syllable as in `squire' for `esquire'

WordNet Word definitions in WordNet
n. the gradual disappearance of an initial (usually unstressed) vowel or syllable as in `squire' for `esquire'

The Collaborative International Dictionary Word definitions in The Collaborative International Dictionary
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.

Wiktionary Word definitions in Wiktionary
n. (context linguistics prosody English) The loss of the initial unstressed vowel of a word.