The term chronostratigraphy refers to that aspect of the field of stratigraphy dealing with temporal (time) relations and ages of rock bodies. Chronostratigraphic classification in the field of stratigraphy organizes rocks on the basis of their age or the time of their genesis.

Chronostratigraphic units are defined as bodies of rock—stratified and non-stratified—that formed during a specific interval of geologic time. Chronostratigraphic units are thus special rock bodies that are conceptual, as well as being material. They can be thought of as the subset of rocks formed during a specified geologic time interval.

For example, the Devonian System is the set of all rocks (sedimentary as well as igneous and metamorphic), wherever they occur on Earth, formed during the Devonian Period. The boundaries of this conceptual set of rocks are synchronous (i.e., are the same age everywhere) and the Devonian System is isochronous (i.e...