A brief stretch of time, and a heavier sense beside it
In ordinary English a moment is a short, loosely bounded stretch of time, or a particular point inside time: wait a moment, at the moment, a defining moment. The same noun still names consequence, as in a matter of great moment, though that use now sounds formal or literary. Physics and statistics keep a third life for the word: a moment as a product of a quantity and a distance from an axis, or as a measure of the shape of a set of values. Those technical senses look remote from everyday speech, but they are closer to the Latin starting point than the clock-face meaning is.
What English did not do is grow the word at home. Old English had other ways to talk about a trice or a turning. Moment is a medieval import.
French and Latin, not a native stem
Accounts of the origin of the word moment agree on Latin mōmentum. They differ on the last hop into English. The Oxford English Dictionary treats the noun as mixed: partly from Old French moment, already in use in the twelfth century with senses of a short time and of weight or value, and partly from the Latin noun itself. The American Heritage Dictionary draws a single line through Middle English and Old French back to Latin. Either path is a borrowing, not inheritance from Germanic.
First dates in English sit in the fourteenth century and do not match to the year. One widely used etymological handbook puts the brief-time sense in the late 1300s, often in phrases of the type moment of time. General dictionaries sometimes open the window earlier, around 1300–1350. The OED’s earliest cited evidence is from before 1382, in the early Wycliffite Bible. The disagreement is small: the word is Middle English, and by the end of that century it is established. Later Scots spellings such as mament, and early rhymes that stress the second syllable, still hint at French and Latin pronunciation rather than the modern English fall of stress on the first syllable.
The Romance family makes the loan obvious. French keeps moment; Spanish, Italian, and Portuguese have momento; German Moment and Dutch moment are the same Latin stock taken into those languages. English later borrowed the Latin form again as momentum, and it already had movement from French mouvement. Those two are doublets of moment: one ancestor, several later shapes and jobs.
A contracted noun from “to move”
Latin mōmentum is usually explained as a contraction of an unattested *movimentum, from the verb movēre, to move, plus the noun suffix -mentum. Later Medieval Latin actually writes uncontracted movimentum, which supports that analysis. The same verb, through Old French, gives English move. Behind it, Indo-European reconstructions cluster around a root of pushing or setting in motion, written in different handbooks as *meue- or *m(y)ewh₁-. Once the contraction is undone, the morphology is ordinary Latin, not an English stem mo- with the later suffix -ment glued on in this language.
Classical Latin already spread the noun across several fields: movement, impulse, and course; change or disturbance; a particle, part, or point; a short space of time; a cause or circumstance; and weight, influence, importance. A thing of great moment is, in that older logic, something that can shift a situation, not only something that occupies a tick of duration.
How the time sense grew out of motion is unsettled. Some handbooks, but not the OED, derive the tiny interval from a particle just heavy enough to move the pointer of a scale, then transfer that smallness from mass to time. The Middle English Dictionary does record moment of a balance for the tongue of a balance, so the weighing image existed in English whether or not it is the true bridge to instant. The cautious version is simpler: Latin already used mōmentum for both a brief interval and for influence, and English took both possibilities in.
From a vague instant to force and a measured hour
The first solid English sense is the short stretch of time. Importance and consequence are recorded from the 1520s. Opportunity, the idea behind seize the moment, is later, from the late eighteenth century. After that the idiom pile grows without changing the etymology: for the moment as a way of saying for now in the 1880s; never a dull moment in the same decade; moment of truth in 1932, taken into English from the Spanish phrase for the last sword-thrust in a bullfight. Careful usage has sometimes distinguished a moment, which has some duration, from an instant, which does not. Everyday speech rarely bothers.
Technical English went back toward moving power. In mechanics a moment is a tendency to produce rotation, or the product of a quantity and its directed distance from a reference: moment of force, moment of inertia. Statistics named moments of a distribution after related products of powers and distances. Those uses sit nearer Latin “moving power” than to wait a second.
Medieval computists also treated a moment as a counted unit, not a shrug. In a scheme described by Bede in the eighth century, a solar hour held four puncta, or ten minuta, or forty momenta. If that hour is a modern sixty-minute hour, the fraction is about ninety seconds. Daylight hours in the older system were often a twelfth of the time from sunrise to sunset, so the moment lengthened in summer and shrank in winter. John Trevisa’s late-fourteenth-century English rendering of Bartholomaeus Anglicus still lays out the ladder, even splitting the moment into ounces and atoms that no sundial of the period could show. Ordinary talk dropped the measured unit. The vague short wait remained.
The twin that kept the motion
The sharpest leftover of the history is the pair moment and momentum. One arrived through French and settled on time and, in higher register, importance. The other was taken straight from Latin and kept the physics of a moving mass. Same contracted noun from movēre; two English words, one still about impetus, the other about a moving instant.