atom

Etymology

From Middle English attome, from Middle French athome, from Latin atomus (“smallest particle”), from Ancient Greek ἄτομος (átomos, “indivisible”), from ἀ- (a-, “not”) + τέμνω (témnō, “I cut”).

noun

1. (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.
   Meronyms: proton, neutron, electron
   Oxygen levels on Earth skyrocketed 2.4 billion years ago, when cyanobacteria evolved photosynthesis: the ability to convert water and carbon dioxide into carbohydrates and waste oxygen using solar energy. The evolutionary precursor of photosynthesis is still under debate, and a new study sheds light. The critical component of the photosynthetic system is the water-oxidizing complex, made up of manganese atoms and a calcium atom. 2013 September–October, Katie L. Burke, “In the news: Photosynthesis precursor”, in American Scientist, archived from the original on 2016-04-13
2. (history of science) A hypothetical particle posited by Greek philosophers as an ultimate and indivisible component of matter.
3. (now generally regarded figurative) The smallest, indivisible constituent part or unit of something.
   Towards the following morning, the thermometer fell to 5°; and at daylight, there was not an atom of water to be seen in any direction. 1835, John Ross, James Clark Ross, “Chapter XXXIV. Labour in Cutting through the Ice—Become Fixed for the Winter—Summary of the Month.”, in Narrative of a Second Voyage in Search of a North-west Passage, and of a Residence in the Arctic Regions, during the Years 1829, 1830, 1831, 1832, 1833; by Sir John Ross, C.B., K.S.A., K.C.S., &c. &c. Captain in the Royal Navy. Including the Reports of Commander (now Captain) J. C. Ross, R.N., F.R.S., F.L.S., &c. and the Discovery of the Northern Magnetic Pole, Philadelphia, Pa.: E. A. Carey & A. Hart; Baltimore, Md.: Carey, Hart & Co., →OCLC, pages 283–284
   But at this critical moment the pirate astern sent a mischievous shot and knocked one of the men to atoms at the helm. 1863, Charles Reade, Hard Cash
4. (philosophy) In logical atomism, a fundamental fact that cannot be further broken down.
5. (historical) The smallest medieval unit of time, equal to fifteen ninety-fourths of a second.
6. A mote of dust in a sunbeam.
7. A very small amount; a whit.
   "Doctor, tell me one word more," said Theodore, quivering with suppressed emotion. "How do you think it will end?" / "I have hardly the faintest atom of hope," answered this honest, earnest man. 1873, “Pansy” [pseudonym; Isabella Macdonald Alden], “A Double Crisis”, in Three People, Cincinnati, Oh.: Western Tract and Book Society, 176 Elm Street, →OCLC, page 325
8. (computing, programming, Lisp) An individual number or symbol, as opposed to a list; a scalar value.
9. (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").
   In a Venn diagram, an atom is depicted as an area circumscribed by lines but not cut by any line.
10. (mathematics, set theory) An element of a set that is not itself a set; an urelement.
11. (Canada, usually attributive) An age group division in hockey for nine- to eleven-year-olds.