The Collaborative International Dictionary of English v.0.48:

Definite \Def"i*nite\, a. [L. definitis, p. p. of definire: cf.
   F. d['e]fini. See Define.]
   1. Having certain or distinct; determinate in extent or
      greatness; limited; fixed; as, definite dimensions; a
      definite measure; a definite period or interval.
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            Elements combine in definite proportions. --Whewell.
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   2. Having certain limits in signification; determinate;
      certain; precise; fixed; exact; clear; as, a definite
      word, term, or expression.
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   3. Determined; resolved. [Obs.] --Shak.
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   4. Serving to define or restrict; limiting; determining; as,
      the definite article.
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   Definite article (Gram.), the article the, which is used to
      designate a particular person or thing, or a particular
      class of persons or things; -- also called a definitive.
      See Definitive, n. -

   Definite inflorescence. (Bot.) See Determinate
      inflorescence, under Determinate.

   Law of definite proportions (Chem.), the essential law of
      chemical combination that every definite compound always
      contains the same elements in the same proportions by
      weight; and, if two or more elements form more than one
      compound with each other, the relative proportions of each
      are fixed. Compare Law of multiple proportions, under
      Multiple.
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WordNet (r) 3.0 (2006):

law of definite proportions
    n 1: (chemistry) law stating that every pure substance always
         contains the same elements combined in the same proportions
         by weight [syn: law of constant proportion, law of
         definite proportions]