Period is the geochronologic equivalent of a System.
A system is a unit of major rank in the conventional chronostratigraphic hierarchy, above a series and below an erathem.
The names of currently recognized systems are of diverse origin inherited from early classifications: some indicate
chronologic position (Tertiary, Quaternary), others have lithologic connotation (Carboniferous, Cretaceous),
others are tribal (Ordovician, Silurian) and still others are geographic (Devonian, Permian).