late 14c., "very brief portion of time, instant," in moment of time, from Old French moment (12c.) "moment, minute; importance, weight, value" and directly from Latin momentum "movement, motion; moving power; alteration, change;" also "short time, instant" (also source of Spanish, Italian momento), contraction of *movimentum, from movere "to move" (from PIE root meue- "to push away").
Some (but not OED) explain the sense evolution of the Latin word by notion of a particle so small it would just "move" the pointer of a scale, which led to the transferred sense of "minute time division."
In careful use, a moment has duration, an instant does not. The sense of "notable importance, 'weight,' value, consequence" is attested in English from 1520s. Meaning "opportunity" (as in seize the moment) is from 1781.
In for the moment "temporarily, so far as the near future is concerned" (1883) it means "the present time." Phrase never a dull moment is attested by 1885 (Jerome K. Jerome, "On the Stage - and Off"). Phrase moment of truth first recorded 1932 in Hemingway's "Death in the Afternoon," from Spanish el momento de la verdad, the final sword-thrust in a bull-fight.
来自拉丁语momentum。运动，变化，冲击，短时间，瞬间，缩写自movimentum,来自movere,运动，移动，词义引申为“片刻，瞬间”，因为事物的不断运动才会不断变化，因为不断的动态的变化才会有“瞬间、转瞬即逝”的概念，如果事物一直静止不变，不可能有“瞬间、转瞬即逝”的说法。词源同move, movement。