The Collaborative International Dictionary of English v.0.48:

Taxable \Tax"a*ble\, a.
   1. Capable of being taxed; liable by law to the assessment of
      taxes; as, taxable estate; taxable commodities.
      [1913 Webster]

   2. (Law) That may be legally charged by a court against the
      plaintiff of defendant in a suit; as, taxable costs.
      [1913 Webster] -- Tax"a*ble*ness, n. -- Tax"a*bly,
      adv.
      [1913 Webster]

WordNet (r) 3.0 (2006):

taxable
    adj 1: (of goods or funds) subject to taxation; "taxable
           income"; "nonexempt property" [syn: taxable,
           nonexempt] [ant: exempt, nontaxable]