Epoch

An epoch is a unit or reference point used to designate a span of time distinguished by particular features or events. In common usage, it marks a significant period in history or development. The word comes from Greek epochē, meaning “point in time.”

Geology and paleontology use the term to subdivide the geologic timescale. An epoch is a subdivision of a period, used to denote relatively shorter intervals within a larger era. Examples in the Cenozoic era include the Paleocene, Eocene, Oligocene, Miocene, Pliocene, Pleistocene, and Holocene. Epoch boundaries are defined by characteristic fossil assemblages and other stratigraphic markers, and their lengths vary by region and convention.

In astronomy, an epoch provides a reference moment for celestial coordinates. Because the positions of objects change over time due to precession and proper motion, coordinates are specified with respect to a particular epoch, such as J2000.0. Observations may need to be expressed or transformed from one epoch to another to maintain accuracy.

In computing and data science, the term appears in several senses. The Unix epoch is the fixed starting point of time: January 1, 1970 UTC, from which time is measured in seconds. In machine learning, an epoch is one complete pass through the training dataset, typically repeated many times during learning.

The term also appears in historical and cultural contexts to denote a notable era or phase of change, such as an epoch of modernization or a postwar epoch. The concept is related to, but distinct from, “era” and “period.”