Synonym: Chronostratic unit
A chronostratigraphic unit is a geologic unit that includes all of the rocks that are formed during a specific interval of geologic time. Chronostratigraphic units are ranked according to the length of time they record into eonothems (the longest), erathems, systems, series, stages and substages. The upper and lower boundaries of a chronostratigraphic unit are defined by the Global Boundary Stratotype Section and Point (GSSP), and the boundary between two chronostratigraphic units is theoretically a synchronous surface. The GSSP is the most important means to determine the synchronous interface.