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A 'Global Standard Stratigraphic Age', often abbreviated 'GSSA', is a chronological age used to define the boundaries between different periods or epochs on the geologic time scale.
GSSAs are defined by the International Commission on Stratigraphy and are only used for time older that 630 million years ago. For more recent periods, a Global Boundary Stratotype Section and Point (GSSP) is used to define such boundaries. In contrast to GSSAs, GSSPs are based on important events and transitions within a particular stratigraphic section. In older sections, there is insufficient fossil record or well preserved sections to identify the key events necessary for a GSSP so GSSAs are defined based on fixed dates.