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.
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:
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 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.
In natural science, there is need for another time perspective, independent from human activity, and indeed spanning a far longer period (mainly prehistoric), where era refers to well-defined time spans which subdivide Eons.
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).
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.
Like epoch, "era" in English originally meant "the starting point of an age"; the meaning "system of chronological notation" is c.1646; that of "historical period" is 1741.