aphetism

from The Century Dictionary.

• noun An aphetized form of a word; a form resulting from the loss of a weak initial vowel, as down for adown.

from the GNU version of the Collaborative International Dictionary of English.

• noun An aphetized form of a word.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The aphetized form of a word, i.e. a word which has dropped its initial vowel or syllable. For example, see squire.
• noun aphesis