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Era is a word used in English since 1615, derived from Late Latin æra, era "an era or epoch from which time is reckoned," probably identical to Latin æra "counters used for calculation," plural of æs "brass, money".

The Latin word's use in chronology seems to have begun in 5th century Visigothic Spain, where it appears in the History of Isidore of Seville, and in later texts. The Spanish era is calculated from 38 BC, perhaps because of a tax (cfr. indiction) levied in that year, or due to a miscalculation of the Battle of Actium, which occurred in 31 BC.

Like epoch, "era" in English it originally meant "the starting point of an age;" the meaning "system of chronological notation" is c.1646; that of "historical period" is 1741.

In chronology, an era is the highest level for the organization of the measurement of time. A calendar era span of many years which are numbered beginning at a specific epoch, which often marks the origin of a state or cosmology, the birth of a leader, or another significant historical or mythological event; it can be called after either accordingly.

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. Such rather impractical system — a nightmare for historians once a single piece of the puzzle is missing — is the use of regnal years, which often reflects the preponderance in public life of the absolute ruler in many ancient cultures, while such tradition sometimes outlives the political power of the throne.

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:

*Chinese Eras
*Japanese Eras
*Korean Eras
*Vietnamese Eras

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. was used since long, but Acts of Parliament used to be 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.

In common speech and various contexts, the term era is also used, by extension, for any (as a rule relatively long) period in history with a name, often relating to common characteristic(s), even if this is not the normal way to organize time.
The most relevant type are politic periods, for example: the Roman era, the Elizabethan era, the Victorian era (dynastic criteria, only formally correct within the British realm/empire/Commonwealth) and the Soviet era, or comparable literary notions like the Biblical era.

The word Era is also popularly used to denote the passing of — often shorter — periods that are only defined in terms of a specific discipline of sphere of life, such as the prominence of an artistic style, or more specifically in music, see musical eras, described in History of music, such as the Big Band era, Disco era. An event such as the death of Frank Sinatra is poetically called the end of an era.

* In natural science, there is need for another time perspective, independent from human activity, and indeed spanning a far longer period (mainly prehistoric), as in Geology where era refers to four well defined time spans covering the entire existence of the planet Earth: from oldest to youngest, these are the Proterozoic, Paleozoic, Mesozoic, and Cenozoic eras, each subdivided in shorter periods; see Geologic timescale for a slightly different interpretation and details.
In astronomy the periods are even longer, to cover the entire existence of the universe (in the order of 13.7 billion years), but usually just denoted in numerical units, as there is no significant link to any earthly reality, our planet being astronomically insignificant (except as the only known observation point).

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