The Tertiary was an official geological period, but it is no longer used. It included the first part of the Cainozoic. This is now divided into the Palaeogene and the Neogene.
The previous use of the term "Tertiary" explains why the last period of the Cainozoic is called the Quarternary. There was a big debate about that. These decisions are made by the International Union of Geological Sciences (IUGS). This organisation runs an International Commission on Stratigraphy, which decides how we should name geological strata and stages.
In 19th and 20th century scientific literature, Tertiary is used where geologists now use Palaeogene and Neogene.
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