atom
n//ˈætəm//atom
(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. [from 16th c.]
(history of science) A hypothetical particle posited by Greek philosophers as an ultimate and indivisible component of matter. [from 15th c.]
(now generally regarded, _, figuratively) The smallest, indivisible constituent part or unit of something. [from 17th c.]
(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"). [from 20th c.]