The Collaborative International Dictionary of English v.0.48:

Flexible \Flex"i*ble\, a. [L. flexibilis: cf. F. flexible.]
   1. Capable of being flexed or bent; admitting of being
      turned, bowed, or twisted, without breaking; pliable;
      yielding to pressure; not stiff or brittle.
      [1913 Webster]

            When the splitting wind
            Makes flexible the knees of knotted oaks. --Shak.
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   2. Willing or ready to yield to the influence of others; not
      invincibly rigid or obstinate; tractable; manageable;
      ductile; easy and compliant; wavering.
      [1913 Webster]

            Phocion was a man of great severity, and no ways
            flexible to the will of the people.   --Bacon.
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            Women are soft, mild, pitiful, and flexible. --Shak.
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   3. Capable or being adapted or molded; plastic,; as, a
      flexible language.
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            This was a principle more flexible to their purpose.
                                                  --Rogers.

   Syn: Pliant; pliable; supple; tractable; manageable; ductile;
        obsequious; inconstant; wavering. -- Flex"i*ble*ness,
        n. -- Flex"i*bly, adv.
        [1913 Webster]

WordNet (r) 3.0 (2006):

flexibleness
    n 1: the property of being flexible; easily bent or shaped [syn:
         flexibility, flexibleness] [ant: inflexibility,
         inflexibleness]
    2: the quality of being adaptable or variable; "he enjoyed the
       flexibility of his working arrangement" [syn: flexibility,
       flexibleness] [ant: inflexibility, rigidity,
       rigidness]