Origins
The word 'cycle' is one of the English language's most direct connections to the technological revolution that transformed prehistoric human civilization: the invention of the wheel. It entered English in the fourteenth century from Late Latin 'cyclus,' borrowed from Greek 'kýklos' (circle, wheel, ring). The Greek word descends from PIE *kʷékʷlos, a reduplicated form based on the root *kʷel- (to revolve, to turn, to move around).
The PIE word *kʷékʷlos holds a special place in historical linguistics because it helps establish when Proto-Indo-European was spoken. The wheel was invented around 3500 BCE in the Pontic-Caspian steppe region. Since cognates of *kʷékʷlos appear in nearly every branch of the Indo-European family — Greek 'kýklos,' Sanskrit 'cakrá' (wheel, circle, the source of 'chakra'), Old English 'hwēol' (wheel), Old Church Slavonic 'kolo' (wheel, circle), Tocharian 'kukäl' (wagon) — the word must have been part of the common Proto-Indo-European vocabulary. This means PIE was spoken as a unified language no earlier than approximately 3500 BCE, since the speakers could not have had a word for a technology that did not yet exist.
The phonological development from PIE *kʷékʷlos to its various descendants illustrates the regular sound changes that define each branch. In Greek, the labiovelar *kʷ became 'k' before back vowels, producing 'kýklos.' In Germanic, *kʷ became 'hw' (later 'wh'), so *kʷékʷlos evolved into Proto-Germanic *hwehwlaz and eventually Old English 'hwēol,' Modern English 'wheel.' In Sanskrit, *kʷ became 'c' (pronounced 'ch'), giving 'cakrá.' These are all regular reflexes of the same ancestral sound.
Latin Roots
The semantic range of 'cycle' in English spans the literal and the abstract. The literal sense of 'circle' or 'circular movement' is closest to the Greek original. From this, the meaning extended to any regularly repeating sequence: the cycle of the seasons, the water cycle, business cycles, life cycles. The verb 'to cycle' meaning 'to ride a bicycle' dates from the 1880s, when the bicycle craze swept Europe and America. 'Bicycle' itself is a nineteenth-century coinage from the Latin prefix 'bi-' (two) and Greek 'kýklos' (circle/wheel) — literally 'two-wheeler.'
Greek 'kýklos' generated numerous English compounds. 'Cyclone' was coined in 1848 by the British meteorologist Henry Piddington, from Greek 'kyklōn' (moving in a circle). 'Encyclopedia' comes from Greek 'enkýklios paideía' (circular/complete education) — an education that comes full circle, covering all subjects. 'Recycle' (1926) means to cycle again through a process. 'Cyclops,' the one-eyed giant of Greek mythology, literally means 'circle-eye' or 'round-eye' (from 'kýklos' + 'ōps,' eye).
The concept of cyclical time versus linear time has been a fundamental philosophical distinction across cultures. Greek and Hindu thought emphasized cyclical time — the Great Year, the Yugas, the eternal return. Judeo-Christian thought introduced a more linear conception: creation, fall, redemption, apocalypse. The English word 'cycle' bridges these traditions, available for both the astronomer's orbital cycles and the economist's boom-and-bust narratives.
Later History
In music, a 'song cycle' is a group of related songs intended to be performed as a unit (Schubert's 'Winterreise,' Schumann's 'Dichterliebe'). In literature, a 'cycle' denotes a group of stories centered on a common theme or hero (the Arthurian cycle, the Trojan cycle). In both cases, the term implies completeness — a circle of works that encompasses a subject fully.
The root *kʷel- has other English descendants beyond 'cycle' and 'wheel.' 'Pole' (the axis of the earth) comes through Latin 'polus' from Greek 'pólos' (pivot, axis), from the same root. 'Pulley' may derive from a related Greek form. 'Colony' comes from Latin 'colōnus' (farmer, settler), from 'colere' (to cultivate, to till — originally to turn the soil), which may also trace to *kʷel-. The act of revolution — political or mechanical — thus connects etymologically to the most basic motion: turning.