The Collaborative International Dictionary of English v.0.48:

Zero \Ze"ro\, n.; pl. Zerosor Zeroes. [F. z['e]ro, from Ar.
   [,c]afrun, [,c]ifrun, empty, a cipher. Cf. Cipher.]
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   1. (Arith.) A cipher; nothing; naught.
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   2. The point from which the graduation of a scale, as of a
      thermometer, commences.
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   Note: Zero in the Centigrade, or Celsius thermometer, and in
         the R['e]aumur thermometer, is at the point at which
         water congeals. The zero of the Fahrenheit thermometer
         is fixed at the point at which the mercury stands when
         immersed in a mixture of snow and common salt. In
         Wedgwood's pyrometer, the zero corresponds with
         1077[deg] on the Fahrenheit scale. See Illust. of
         Thermometer.
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   3. Fig.: The lowest point; the point of exhaustion; as, his
      patience had nearly reached zero.
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   Absolute zero. See under Absolute.

   Zero method (Physics), a method of comparing, or measuring,
      forces, electric currents, etc., by so opposing them that
      the pointer of an indicating apparatus, or the needle of a
      galvanometer, remains at, or is brought to, zero, as
      contrasted with methods in which the deflection is
      observed directly; -- called also null method.

   Zero point, the point indicating zero, or the commencement
      of a scale or reckoning.
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WordNet (r) 3.0 (2006):

zero point
    n 1: the point on a scale from which positive or negative
         numerical quantities can be measured [syn: zero, zero
         point]