Source : Webster's Revised Unabridged Dictionary (1913)

Deductible \De*duct"i*ble\, a.
   1. Capable of being deducted, taken away, or withdrawn.

            Not one found honestly deductible From any use that
            pleased him.                          --Mrs.
                                                  Browning.

   2. Deducible; consequential.

Source : WordNet®

deductible
     adj : acceptable as a deduction (especially as a tax deduction)
           [ant: {nondeductible}]
     n 1: (taxes) an amount that can be deducted (especially for the
          purposes of calculating income tax)
     2: a clause in an insurance policy that relieves the insurer of
        responsibility to pay the initial loss up to a stated
        amount