epoch

noun

• a period of time in history or a person’s life, typically one marked by notable events or particular characteristics:the Victorian epoch
• the beginning of a distinctive period in the history of someone or something:welfare reform was an epoch in the history of U.S. social policy
• Geology a division of time that is a subdivision of a period and is itself subdivided into ages, corresponding to a series in chronostratigraphy:the Pliocene epoch
• Astronomy an arbitrarily fixed date relative to which planetary or stellar measurements are expressed.