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tertiary (adj.)

1650s, "of the third order, rank, degree, etc.," from Latin tertiarius "of or pertaining to a third," from tertius "third, a third," from root of tres "three" (see three). The geological sense (with capital T-) of "era after the Mesozoic" (which formerly was called the Secondary) is attested from 1794, after Italian terziari, used in this sense 1760 by Italian geologist Giovanni Arduino (1714-1795).

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Definitions of tertiary from WordNet
1
tertiary (adj.)
coming next after the second and just before the fourth in position;
Synonyms: third / rd
2
Tertiary (n.)
from 63 million to 2 million years ago;
Synonyms: Tertiary period
From wordnet.princeton.edu