Origins
The word epoch started life as a full stop. Its Greek ancestor epokhḗ (ἐποχή) meant a stoppage or held moment — the exact opposite of the long stretch of time the English word now describes. The shift from point to period is one of the quieter semantic inversions in the language, and one of the cleanest examples of metonymy: the reference point swallowed the thing being referred to.
The Greek verb underneath is epékhein, to hold upon or to pause, built from epi- (upon) and ékhein (to hold). The noun epokhḗ is attested in this technical sense by the second century BCE in the astronomical writings of Hipparchus and, later, in Ptolemy's Almagest (c. 150 CE), where it names the fixed moment from which planetary positions are calculated. A stargazer who uses an epokhḗ is holding the sky still — freezing the coordinates of a planet at one instant so that every future position can be calculated by applying elapsed time to that frozen reference. It is a reference frame, not a span. Greek astronomy had a separate word, aiṓn, for the long stretch of time, and the two words were not confused before the Latin Middle Ages.
The word had a parallel career in Greek philosophy. The Pyrrhonian skeptics used epokhḗ to mean the suspension of judgement — a held-back moment in which one refuses to commit to either side of a question. Sextus Empiricus, writing in the second century CE, makes epokhḗ the technical term for the skeptic's central move in his Outlines of Pyrrhonism. This philosophical sense survived quietly through the late-antique and medieval periods in commentaries on the skeptics, and re-emerged explosively in twentieth-century phenomenology, where Edmund Husserl's Epoché is the methodological bracketing of natural assumptions — the hesitation that makes philosophy possible. The astronomical and philosophical senses are the same word: both name a held stillness from which something else can be measured or thought.
Latin Roots
The word crossed into Latin as epocha in late medieval astronomy. Joseph Scaliger used it in 1583 in his De Emendatione Temporum, his great work on chronology, to name the fixed starting points (the founding of Rome, the Hijra, the birth of Christ) from which different civilisations counted their years. It entered English in 1614, attested in Sir Walter Raleigh's History of the World, where he writes of the epocha from which the ancients measured their years. For roughly a century it kept the original point sense: an epoch was a fixed moment marking the start of a reckoning. Samuel Johnson's Dictionary of 1755 defines epoch as the time at which a new computation is begun — still firmly a point, not a period. The drift from point to period is largely a nineteenth-century development, driven by geology. When Charles Lyell organised stratigraphy in his Principles of Geology (1830–1833), he needed a vocabulary of nested temporal divisions, and epoch slotted into place below period and above age. The Oxford English Dictionary's first fascicle covering the letter E appeared in 1891; by then the ordinary English sense was firmly the long stretch, and only the technical astronomical usage preserved the original point.
Proto-Indo-European *seǵʰ- (to hold) sits behind all of this. The same root gave Greek skhêma (form — something held in shape), source of English scheme and schematic. It gave hektikós (habitual — a state the body holds), source of hectic, which in early English meant specifically the persistent fever of tuberculosis. In Sanskrit it produced sahate (he endures, he holds out) and sahá (strong one, able). In Avestan hazah- names strength. The Germanic branch contributed Gothic sigis (victory — that which is held), ancestor of the Germanic name-element Sig- (Siegfried, Sigurd). In Baltic it gave Lithuanian sėgti (to fasten, to hold). The semantic field is always one of holding, enduring, maintaining a position. Every word in the family is, in one way or another, about not letting go.
Cross-linguistically the sibling forms preserved the astronomical sense longer than English did. Italian epoca, Spanish época, Portuguese época, and French époque all still carry the double meaning of starting-point and era, though the era sense dominates everyday use — the French Belle Époque is a period, not a point. German Epoche, borrowed through Latin in the seventeenth century, keeps the philosophical Husserlian sense intact in scholarly prose and the ordinary era sense in everyday speech. Russian эпоха (epokha) is a direct Greek-through-Church-Slavonic borrowing, dating to the Kievan scholarly tradition. Modern Greek εποχή still means both the fixed reference and the era — and also, in everyday speech, the season of the year, a fourth sense that comes naturally from the word's held-moment core.
Greek Origins
Computers quietly preserved the original meaning. The Unix epoch — midnight UTC on 1 January 1970 — is exactly what a Greek astronomer would recognise: a fixed reference moment from which all others are counted. Every timestamp in a Unix-derived system is the number of seconds since that held instant. The engineers of Bell Labs who chose it in 1969 were not thinking of Hipparchus, but the choice was a direct revival of the word's two-thousand-year-old technical sense. JavaScript uses the same epoch in milliseconds; NTP time uses 1 January 1900; the IBM mainframe world prefers 1 January 1900 or 1 January 1970 depending on the system. The habit of picking a frozen moment and counting outward from it is, it turns out, older than writing about astronomy. In the mouths of programmers, the word has finally come back around to the meaning it started with: a held instant, not a stretch. Scaliger would approve.