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.
      [1913 Webster]

   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.
      [1913 Webster]

            Women are soft, mild, pitiful, and flexible. --Shak.
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   3. Capable or being adapted or molded; plastic,; as, a
      flexible language.
      [1913 Webster]

            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):

flexible
    adj 1: capable of being changed; "flexible schedules" [ant:
           inflexible]
    2: able to flex; able to bend easily; "slim flexible birches"
       [syn: flexible, flexile] [ant: inflexible]
    3: able to adjust readily to different conditions; "an adaptable
       person"; "a flexible personality"; "an elastic clause in a
       contract" [syn: elastic, flexible, pliable, pliant]
    4: bending and snapping back readily without breaking [syn:
       flexible, whippy]
    5: making or willing to make concessions; "loneliness tore
       through him...whenever he thought of...even the compromising
       Louis du Tillet" [syn: compromising, conciliatory,
       flexible] [ant: inflexible, sturdy, uncompromising]