epoch
definition: a unit of geological time that is a subdivision of a period and is itself divided into ages
example:
speech part: noun
synonyms:
definition: a period marked by distinctive character or reckoned from a fixed point or event
example:
speech part: noun
synonyms:
era
definition: (astronomy) an arbitrarily fixed date that is the point in time relative to which information (as coordinates of a celestial body) is recorded
example:
speech part: noun
synonyms:
date of reference