What is a period/era?

Explanation:

An era is a span of time defined for the purposes of chronology or historiography, as in the regnal eras in the history of a given monarchy, a calendar era used for a given calendar, or the geological eras defined for the history of Earth. Comparable terms are epoch, age, period, saeculum, aeon and Sanskrit yuga.

a period of time considered as being of a distinctive character; epoch. an extended period of time the years of which are numbered from a fixed point or eventthe Christian era.

the chair is the committee member from the majority party who has the most seniority. in the south carolina senate, the chair is the committee member from the majority party who has the most committee seniority.