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.