1610s, "one's share or portion," from Latin quantum (plural quanta) "as much as, so much as; how much? how far? how great an extent?" neuter singular of correlative pronominal adjective quantus "as much" (see quantity). Introduced in physics directly from Latin by Max Planck, 1900; reinforced by Einstein, 1905. Quantum theory is from 1912; quantum mechanics, 1922; quantum jump is first recorded 1954; quantum leap, 1963, often figurative.
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