These words can serve as arbitrary labels given to sequences of time. Period is the most general of these and aeon the least definite. Period can describe any passage of time, great or small: a rest period of five minutes; the stormy period of adolescence. In reference to history, the word can loosely characterize a sequence of time as a convenient aid to discussion, without claiming that such a sequence is homogeneous or self contained: the period of artistic ferment between the two world wars. By contrast, aeon is used, often in the plural, to indicate an immeasurably long stretch of time. While otherwise vague in reference, it does explicitly reject any notion of uniformity in the period alluded to: the aeons before man’s appearance on the planet; the aeons remaining before the sun’s extinction. Cycle is much more clear-cut at its most restricted, referring to a single and complete instance in a recurring pattern of time.

• Acycle of the sun takes a year, while one cycle of the moon, revealing all its phases, takes 28 days; the life cycle of the hookworm.

Sometimes, the word can refer to the recurring pattern itself: the business cycle and its alternation between the bull and bear markets.

Epoch , most strictly, indicates an event of such significance that it can be said to usher in a new period in history: The bombing of Hiroshima marked an epoch in man’s history. More often now, the word is used to refer indefinitely to a compact or self-contained period in human history, one that is recognizable because in the New World. Era refers exclusively to such a self-contained period of history, although the time span implied may be longer than that suggested by the extended use of epoch : Cubism, Abstractionism and Pop Art have all been epochs in the post-realistic era .

Generation refers to the period between the birth of the parents and the birth of their offspring, commonly calculated for human beings in Western countries as 30 years; the word may also refer to the offspring themselves: the first generation of Italians to be born in Australia. More pertinent here, generation has become a fad word for typifying any era, often shorter than 30 years, in which young people coming of age seem involved in a set of concerns or sociological behaviour that is more characteristic of them than of their parents: the Bodgie and Widgie generation of the post-war years; hippies who consider themselves members of the Love Generation. Often the word is used to make dramatic-sounding phrases without necessarily being apt for the members of the whole group so typified. It is widely if loosely used, also, to denote a group of young people at a particular period in time: the present generation of university students.

Age might often suggest a longer lasting period than epoch or era , one that could be typified in terms of some dominant interest or person: the Age of Shakespeare; the Atomic Age ; the Age of Churchill. Some writers feel that age is preferable to era in such designations, since the former is factual in tone, whereas era can possibly sound pretentious or inflated.

In geology, palaeontology and archaeology, the terms era , period , epoch and age have been assigned arbitrary definitions. In palaeontology, the palaeozoic era is divided into the age of invertebrates, the age of fishes, and the age of amphibians. In geology, the palaeozoic era is divided into periods , such as the permian period , which is further subdivided into epochs , such as the Thuringian Epoch . In archaeology, the basic division is an age , determined by the predominant culture, as the Stone Age , the Bronze Age and the Iron Age . The Stone Age is subdivided into periods or eras , as the neolithic and palaeolithic periods or eras .