Tertiary Period, a division of geologic time that began about 65,000,000 years ago and ended about 2,000,000 years ago. The Tertiary Period is the first period of the Cenozoic Era. It was preceded by the Mesozoic Era (sometimes called the Age of Reptiles) and followed by the Quaternary Period (the Age of Man). The Tertiary Period is divided into the Paleocene, Eocene, Oligocene, Miocene, and Pliocene epochs.
In another system of classification, used by some geologists, the Cenozoic Era is divided into the Paleogene and Neogene periods. The Paleogene Period is usually considered to consist of the Paleocene, Eocene, and Oligocene epochs. The Miocene and Pliocene epochs are included in the Neogene Period.
The term tertiary comes from a Latin word meaning third. It dates back to a system of classification, no longer used, in which all of geologic time was divided into four periods—the Primary, Secondary, Tertiary, and Quaternary.