The Collaborative International Dictionary of English v.0.48:

Voidable \Void"a*ble\, a.
   1. Capable of being voided, or evacuated.
      [1913 Webster]

   2. (Law) Capable of being avoided, or of being adjudged void,
      invalid, and of no force; capable of being either avoided
      or confirmed.
      [1913 Webster]

            If the metropolitan . . . grants letters of
            administration, such administration is not, but
            voidable by sentence.                 --Ayliffe.
      [1913 Webster]

   Note: A voidable contract may be ratified and confirmed; to
         render it null and of no effect, it must be avoided; a
         void contract can not be ratified.
         [1913 Webster]

WordNet (r) 3.0 (2006):

voidable
    adj 1: capable of being rescinded or voided; "the judgment was
           rescindable"; "voidable contracts" [syn: rescindable,
           voidable]