In geology, originally the basic unit of geological time in which a single type of rock system is formed. As later used, any intermediate period of geological and biological history of a Terrestrial Class planet, lasting tens to hundreds of millions of years. Periods are grouped into eras, and divided into epoches. Periods are only relevant to that planet the rocks of which they pertain to.
 A historical, or even galactic, unit of time, dividing an era characterized by particular historical, astronomical, or even cosmological events. In some cases the term will be used interchangably with the term "age". Due to the inherently abstract nature of the term, determining the exact beginning or ending to any period is often a matter of debate. Historical ages and periods can be defined by political (ComEmp Period), cultural (Age of Enlightenment), and military (Version War Period) milestones.