Oligocene a. : Of, pertaining to, or designating, certain strata which occupy an intermediate position between the Eocene and Miocene periods.

Oligocene n. : The Oligocene period. See the Chart of Geology.

Example:
What were the major plate tectonic changes during the: Eocene, Oligocene, Miocene?

Taxonomy
Oligocene, fauna, early Oligocene, late Oligocene, climate change, Oligocene epoch, 7M, continents, free encyclopedia, climate, Oceans, Paleogene period, Eocene epoch, epoch, boundary, stepwise, depression, Haines.

Defination from the web:
Oligocene between North America and Europe since the faunas of the two regions are very similar. During sometime in the Oligocene, South America was finally detached from Antarctica and drifted north towards North America. ...