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{language, word, form}
{day, year, event}
{theory, work, human}
{son, year, death}
{math, energy, light}
{country, population, people}
{album, band, music}
{rate, high, increase}
{style, bgcolor, rowspan}
{church, century, christian}

An era is a commonly used word for long period of time. When used in science, for example geology, eras denote clearly defined periods of time of arbitrary but well defined length, such as for example the Mesozoic era from 252 Ma–66 Ma, delimited by a start event and an end event. When used in social history, eras may for example denote a period of some monarch's reign. In colloquial language, eras denote longer spans of time, before and after which the practices or fashions change to a significant degree.



In chronology, an era is the highest level for the organization of the measurement of time. A calendar era indicates a span of many years which are numbered beginning at a specific reference date (epoch), which often marks the origin of a political state or cosmology, dynasty, ruler, the birth of a leader, or another significant historical or mythological event; it is generally called after its focus accordingly as in Victorian era.

Regnal eras

The word era also denotes the units used under a different, more arbitrary system where time is not represented as an endless continuum with a single reference year, but each unit starts counting from one again, as if time starts again. The use of regnal years is a rather impractical system, and a nightmare for historians if a single piece of the puzzle is missing, and often reflects the preponderance in public life of an absolute ruler in many ancient cultures. Such traditions sometimes outlive the political power of the throne, and may even be based on mythological events or rulers who may not have existed (for example Rome numbering from the rule of Romulus and Regulus). In a manner of speaking the use of the supposed date of the birth of Christ as a base year is a form of a Regnal era.

In East Asia, each emperor's reign may be subdivided into several reign periods, each being treated as a new era. The name of each was a motto or slogan chosen by the emperor. Different East Asian countries utilized slightly different systems, notably:

A similar practice survived in the United Kingdom until quite recently, but only for formal official writings: in daily life the ordinary year A.D. has been used for a long time, but Acts of Parliament were dated according to the years of the reign of the current Monarch, so that "61 & 62 Vict c. 37" refers to the Local Government (Ireland) Act 1898 passed in the session of Parliament in the 61st/62nd year of the reign of Queen Victoria.

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