epoch

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A particular period of history, especially one considered remarkable or noteworthy.
2. n. A notable event that marks the beginning of such a period. See Synonyms at period.
3. n. A unit of geologic time that is a division of a period.
4. n. Astronomy An instant in time that is arbitrarily selected as a point of reference.

Century Dictionary and Cyclopedia

1. n. A point of time from which succeeding years are numbered; especially, a point of time distinguished by some remarkable event, or the event itself as distinguishing the time of its occurrence.
2. n. Hence A specific period of time; any space of time considered as a unit with reference to some particular characteristic or course of events.
3. n. In geology, specifically, one of the shorter divisions of geological time. This word is used differently by different geological writers. Thus, Jukes divides the entire series of fossiliferous strata into only three epochs, while Dana makes eight out of the Lower Silurian alone. Some later writers avoid the use of such words as epoch and age, saying, for instance, instead of Silurian epoch or age, simply Silurian.
4. n. In astronomy, an arbitrary fixed date, for which the elements of a planetary or cometary orbit, or of any motion, are given.
5. n. See the adjectives.
6. n. See equivalent phrases under era.
7. n. Synonyms Epoch, Era, Period, Age. Epoch and era should be distinguished, though in common usage they are interchanged. “An era is a succession of time: an epoch is a point of time. An era commonly begins at an epoch. We live in the Christian era, in the Protestant era, in the era of liberty and letters. The date of the birth of Christ was an epoch: the period of the dawn of the Reformation was an epoch” (A. Phelps, Eng. Style, p. 365). Period may be the opposite of epoch, in being the date at which anything ends, or it may be mere duration, or duration from point to point; the word is very free and often indefinite in its range of meaning. The meaning of age is modified by its connection with human life, so as often to be associated with a person: as, the age of Pericles; but it is also freely applied to time, viewed as a period of some length: as, the bronze age; the golden age; this is an age of investigation.
8. n. In the mechanics of vibration, a term introduced into the equation for a simple harmonic motion in cases where time is not reckoned from the instant when the vibrating particle has reached its greatest positive elongation. The equation then becomes x = α cos (tω + e), in which e is the epoch. The epoch is the angle traversed by the point of reference in the interval between the era of reckoning and the instant of greatest elongation.

Wiktionary

1. n. A particular period of history, especially one considered remarkable or noteworthy.
2. n. A notable event which marks the beginning of such a period.
3. n. astronomy A precise instant of time that is used as a reference point.
4. n. computing, uncountable A precise instant of time that is used as a reference point (e.g. January 1, 1970, 00:00:00 UTC).

GNU Webster's 1913

1. n. A fixed point of time, established in history by the occurrence of some grand or remarkable event; a point of time marked by an event of great subsequent influence
2. n. A period of time, longer or shorter, remarkable for events of great subsequent influence; a memorable period.
3. n. (Geol.) A division of time characterized by the prevalence of similar conditions of the earth; commonly a minor division or part of a period.
4. n. The date at which a planet or comet has a longitude or position.
5. n. An arbitrary fixed date, for which the elements used in computing the place of a planet, or other heavenly body, at any other date, are given

WordNet 3.0

1. n. (astronomy) an arbitrarily fixed date that is the point in time relative to which information (as coordinates of a celestial body) is recorded
2. n. a unit of geological time that is a subdivision of a period and is itself divided into ages
3. n. a period marked by distinctive character or reckoned from a fixed point or event