Epoch (reference date)  

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In the fields of chronology and periodization, an epoch means an instant in time chosen as the origin of a particular era. The "epoch" then serves as a reference point from which time is measured. Time measurement units are counted from the epoch so that the date and time of events can be specified unambiguously.

Events taking place before the epoch can be dated by counting negatively from the epoch, though in pragmatic periodization practice, epochs are defined for the past, and another epoch is used to start the next era, therefore serving as the ending of the older preceding era. The whole purpose and criteria of such definitions is to clarify and co-ordinate scholarship about a period, at times, across disciplines.

Epochs are generally chosen to be convenient or significant by a consensus of the time scale's initial users, or by authoritarian fiat. The epoch moment or date is usually defined by a specific clear event, condition, or criteria— the epoch event or epoch criteria —from which the period or era or age is usually characterized or described.

Examples:

by events:

The assassination of the Roman Emperor Alexander Severus triggering the Crisis of the Third Century

The defenestration of Prague triggering the Thirty Years' War

Queen Victoria ascending to the throne giving the start of the Victorian era

by criteria:

The spurt in exploration, mercantilism, and colonization in the Age of Discovery

Particular ratios of animal fossils in a rock strata —various Geology epochs