From HomoExcelsior

1. A part or particle considered to be an irreducible constituent of a specified system. 2. The irreducible, indestructible material unit postulated by ancient atomism. 3. An extremely small part, quantity, or amount. 4.  centimeter and characteristically remains undivided in chemical reactions except for limited removal, transfer, or exchange of certain electrons. 5. This unit regarded as a source of nuclear energy.

---