Source : Webster's Revised Unabridged Dictionary (1913)

Opposable \Op*pos"a*ble\, a.
   1. Capable of being opposed or resisted.

   2. Capable of being placed opposite something else; as, the
      thumb is opposable to the forefinger.

Source : WordNet®

opposable
     adj : capable of being placed opposite to something; "the thumb is
           opposable to the forefinger" [syn: {apposable}] [ant: {unopposable}]