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    adj 1: capable of being rescinded or voided; "the judgment was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]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

VOIDABLE. That which has some force or effect, but which, in consequence of
some inherent quality, may be legally annulled or avoided.
     2. As a familiar example, may be mentioned the case of a contract, made
by an infant with an adult, which maybe avoided or confirmed by the former
on his coining of age. Vide Parties, contracts.
     3. Such contracts are generally of binding force until avoided by the
party having a right to annul them. Bac. Ab. Infancy, 1 3; Com. Dig. Enfant;
Fonb. Eq. b. 1, c. 2, Sec. 4, note b; 3 Burr. 1794 Nels. Ch. R. 5 5; 1 Atk.
3 5 4; Str. 9 3 7; Perk. Sec. 12. VOIR. An old French word, which signifies
the same as the modern word vrai, true. Voir dire, to speak truly, to tell
the truth.
     2. When a witness is supposed to have an interest in the cause, the
party against whom he is called has the choice to prove such interest by
calling another witness to that fact, or be may require the witness produced
to be sworn on his voir dire as to whether he has an interest in the cause,
or not, but the party against whom he is called will not be allowed to have
recourse to both methods to prove the witness interest. If the witness
answers he has no interest, he is competent, his oath being conclusive; if
he swears he has an interest, he will be rejected.
     3. Though this is the rule established beyond the power of the courts
to change, it seems not very satisfactory. The witness is sworn on his voir
dire to ascertain whether he has an interest, which would disqualify him,
because he would be tempted to perjure himself, if he testified when
interested. But when he is asked whether he has such an interest, if he is
dishonest and anxious to be sworn in the case, he will swear falsely he has
none, and his answer being conclusive, he will be admitted as competent; if,
on the contrary, he swears truly he has an interest, when he knows that will
exclude him, he is told that for being thus honest, he must be rejected.
See, generally, 12 Vin. Ab. 48; 22 Vin. Ab. 14; 1 Dall, 375; Dane's Ab.
Index, h.t.; and Interest.