In geology, a subdivision of Epochs. Ages are the smallest recognized division of geologic time, lasting several million years or less.
 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 often be used interchangably with the term period. Due to the inherently abstract nature of the term, determining the exact beginning or ending to any age is often a matter of debate. Historical ages and periods can be defined by political (ComEmp Period), cultural (Age of Enlightenment), military (Version War Period), and technological milestones.
- Epoch (Geology) - Text by M. Alan Kazlev and Mark Ryherd
A division of geological time, lasting several million years or less. Epochs are grouped into periods, and divided into ages.
- Era - Text by M. Alan Kazlev
 Two or more geological periods. An era may be hundreds of millions of years in duration, and is defined by particular geological or biological processes.
 An extended historical, or even galactic, period of time, that is characterized by particular historical, astronomical, or even cosmological events.
Text by Mark Ryherd
Initially published on 16 January 2010.