The Collaborative International Dictionary of English v.0.48:

Income \In"come\, n.
   1. A coming in; entrance; admittance; ingress; infusion.
      [Obs.] --Shak.
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            More abundant incomes of light and strength from
            God.                                  --Bp. Rust.
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            At mine income I louted low.          --Drant.
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   2. That which is caused to enter; inspiration; influence;
      hence, courage or zeal imparted. [R.]
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            I would then make in and steep
            My income in their blood.             --Chapman.
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   3. That gain which proceeds from labor, business, property,
      or capital of any kind, as the produce of a farm, the rent
      of houses, the proceeds of professional business, the
      profits of commerce or of occupation, or the interest of
      money or stock in funds, etc.; revenue; receipts; salary;
      especially, the annual receipts of a private person, or a
      corporation, from property; as, a large income.
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            No fields afford
            So large an income to the village lord. --Dryden.
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   4. (Physiol.) That which is taken into the body as food; the
      ingesta; -- sometimes restricted to the nutritive, or
      digestible, portion of the food. See Food. Opposed to
      output.
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   Income bond, a bond issued on the income of the corporation
      or company issuing it, and the interest of which is to be
      paid from the earnings of the company before any dividends
      are made to stockholders; -- issued chiefly or exclusively
      by railroad companies.

   Income tax, a tax upon a person's incomes, emoluments,
      profits, etc., or upon the excess beyond a certain amount.

   Syn: Gain; profit; proceeds; salary; revenue; receipts;
        interest; emolument; produce.
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WordNet (r) 3.0 (2006):

income tax
    n 1: a personal tax levied on annual income

Moby Thesaurus II by Grady Ward, 1.0:

54 Moby Thesaurus words for "income tax":
   VAT, ad valorem duty, alcohol tax, amusement tax,
   assessment on default, capital gains tax, capitation,
   capitation tax, corporation tax, customs, customs duty, death duty,
   death tax, doomage, duty, estate duty, estate tax,
   excess profits tax, excise, excise tax, export tax, federal tax,
   gabelle, gift tax, head tax, import tax, inheritance tax,
   internal revenue tax, land tax, liquor tax, local tax, luxury tax,
   nuisance tax, personal property tax, poll, poll tax, property tax,
   property-increment tax, protective tariff, provincial tax, rates,
   revenue tariff, sales tax, salt tax, school tax, severance tax,
   specific duty, state tax, tariff, tariff duty, telephone tax,
   use tax, value added tax, window tax