1. Any great time period in the earth's history marked by special phases of physical conditions or organic development.
2. A formal geologic unit of time which corresponds to a stage.
3. An informal geologic time unit that corresponds to any stratigraphic unit.
4. The fourth-order division of geologic time, which establishes the limits or boundaries of by very minor changes; usually, in sea level and/or climate or in the biota, and usually local.

This is limited to a single continent or a part of it.