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.

6. A mote of dust in a sunbeam.

7. A very small amount; a whit.

8. (Lisp) An individual number or symbol, as opposed to a list; a scalar value.

9. A non-zero member of a Boolean algebra that is not a union of any other elements. Or, a non-zero member of a Boolean lattice that has only zero below it.

   "In a Venn diagram, an atom is depicted as an area circumscribed by lines but not cut by any line."

10. An element of a set that is not itself a set; an urelement.

11. (usually capitalised as "Atom") A member of an age group division in hockey for ten- to 11-year-olds.