A geological period is one of the several subdivisions of geologic time enabling cross-referencing of rocks and geologic events from place to place. It is the fundamental unit in the hierarchy of divisions into which geologists have split the Earth's history. Periods are combined into eras and may themselves be subdivided into epochs.
The rocks formed during a period belong to a chronostratigraphic unit called a system.
The twelve currently recognised periods of the present eon – the Phanerozoic – are defined by the International Commission on Stratigraphy (ICS) by reference to the stratigraphy at particular locations around the world. In 2004 the Ediacaran Period of the latest Precambrian was defined in similar fashion, and was the first such newly designated period in 130 years.
A consequence of this approach to the Phanerozoic periods is that the ages of their beginnings and ends can change from time to time as the absolute age of the chosen rock sequences, which define them, is more precisely determined.
The set of rocks (sedimentary, igneous or metamorphic) that formed during a geological period is known as a system; for example, the 'Jurassic System' of rocks was formed during the 'Jurassic Period' (between 201 and 145 million years ago).
The following table includes all currently recognized periods. The table omits the time before 2500 million years ago, which is not divided into periods.
In a steady effort ongoing since 1974, the International Commission on Stratigraphy has been working to correlate the world's local stratigraphic record into one uniform planet-wide benchmarked system.
American geologists have long considered the Mississippian and Pennsylvanian to be periods in their own right though the ICS now recognises them both as 'subperiods' of the Carboniferous Period recognised by European geologists. Cases like this in China, Russia and even New Zealand with other geological eras has slowed the uniform organization of the stratigraphic record.