apocope άποκοπή
a-poc'-o-pe from Gk. apo “away from” and
koptein “to cut” ("a cutting off")
abissio, abscissio, abcisio, or absissio
cutting from the end
 

  Omitting a letter or syllable at the end of a word. A kind of metaplasm.
Examples
 

Omission of a final letter:
When Maro says "Achilli" for "Achillis"

Omission of a final syllable:
True art is nature to advantage dressed
What oft was thought, but ne'er so well expressed. [for "often"]
—Alexander Pope

Season your admiration for awhile With an attent ear. [for "attentive"]
—Shakespeare, Hamlet 1.2.192