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(chemistry, physics) The smallest possible amount of matter which still retains its identity as a chemical element, now known to consist of a nucleus surrounded by electrons.
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(history of science) A hypothetical particle posited by Greek philosophers as an ultimate and indivisible component of matter.
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(now generally regarded figuratively) The smallest, indivisible constituent part or unit of something.
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(philosophy) In logical atomism, a fundamental fact that cannot be further broken down.
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(historical) The smallest medieval unit of time, equal to fifteen ninety-fourths of a second.
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A mote of dust in a sunbeam.
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A very small amount; a whit.
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(computing, programming, Lisp) An individual number or symbol, as opposed to a list; a scalar value.
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(mathematics) A non-zero member of a partially ordered set that has only zero below it (assuming that the poset has a least element, its "zero").
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(mathematics, set theory) An element of a set that is not itself a set; an urelement.
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(Canada, usually attributive) An age group division in hockey for nine- to eleven-year-olds.