Atom

noun

1. 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.
2. (history of science) A hypothetical particle posited by Greek philosophers as an ultimate and indivisible component of matter.
3. The smallest, indivisible constituent part or unit of something.
4. In logical atomism, a fundamental fact that cannot be further broken down.
5. The smallest medieval unit of time, equal to fifteen ninety-fourths of a second.