n The smallest, indivisible constituent part or unit of something.
n (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.
n (mathematics) A non-zero member of a (w|Boolean algebra) that is not a union of any other elements.
n (historical) A theoretical particle of matter, imagined to be incapable of further division; the smallest possible unit of substance.
n (obsolete) The smallest medieval unit of time, equal to fifteen ninety-fourths of a second.