epoch

noun

1. the beginning of a new and important period in the history of anything: the first earth satellite marked a new epoch in the study of the universe
2. a period of time considered in terms of noteworthy and characteristic events, developments, persons, etc.: an epoch of social revolution
3. a point in time or a precise date
4. Astron. the time at which observations are made, as of the positions of planets or stars
5. Geol. a subdivision of a period in geologic time corresponding to the rock strata of a series ()