1. periodGeology, Abbreviation
2. Nihon-shokiAbbreviation, See also 日本書紀
Wikipedia definition
3. System (stratigraphy)A system in the natural sciences and stratigraphy is an idealized composite unit of the geologic record made up of a succession of rock layers that were laid down together within a certain corresponding geological time span, and are used in turn to date things to a certain corresponding geologic period.
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