The Collaborative International Dictionary of English v.0.48:

nontaxable \nontaxable\ adj.
   Not subject to taxation; -- of goods imported into a country
   or sold at retail outlets; as, most laws imposing sales taxes
   make food nontaxable. Opposite of taxable. [Narrower terms:
   duty-free; tax-exempt, tax-free; unratable; untaxed ]
   Also See: {exempt.
   [WordNet 1.5]