The Collaborative International Dictionary of English v.0.48:

Tertiary \Ter"ti*a*ry\, a. [L. tertiarius containing a third
   part, fr. tertius third: cf. F. tertiaire. See Tierce.]
   1. Being of the third formation, order, or rank; third; as, a
      tertiary use of a word. --Trench.
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   2. (Chem.) Possessing some quality in the third degree;
      having been subjected to the substitution of three atoms
      or radicals; as, a tertiary alcohol, amine, or salt. Cf.
      Primary, and Secondary.
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   3. (Geol.) Later than, or subsequent to, the Secondary.
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   4. (Zool.) Growing on the innermost joint of a bird's wing;
      tertial; -- said of quills.
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   Tertiary age. (Geol.) See under Age, 8.

   Tertiary color, a color produced by the mixture of two
      secondaries. "The so-called tertiary colors are citrine,
      russet, and olive." --Fairholt.

   Tertiary period. (Geol.)
      (a) The first period of the age of mammals, or of the
          Cenozoic era.
      (b) The rock formation of that period; -- called also
          Tertiary formation. See the Chart of Geology.

   Tertiary syphilis (Med.), the third and last stage of
      syphilis, in which it invades the bones and internal
      organs.
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WordNet (r) 3.0 (2006):

Tertiary period
    n 1: from 63 million to 2 million years ago [syn: Tertiary,
         Tertiary period]