a period marked by distinctive character or reckoned from a fixed point or event

(astronomy) an arbitrarily fixed date that is the point in time relative to which information (as coordinates of a celestial body) is recorded

a unit of geological time that is a subdivision of a period and is itself divided into ages

highly significant or important especially bringing about or marking the beginning of a new development or era

highly significant or important especially bringing about or marking the beginning of a new development or era