at·om (ătˈəm)

1. A unit of matter, the smallest unit of an element, having all the characteristics of that element and consisting of a dense, central, positively charged nucleus surrounded by a system of electrons. The entire structure has an approximate diameter of 10^-8 centimeter and characteristically remains undivided in chemical reactions except for limited removal, transfer, or exchange of certain electrons.
2. This unit regarded as a source of nuclear energy.
3. A part or particle considered to be an irreducible constituent of a specified system.
4. The irreducible, indestructible material unit postulated by ancient atomism.
5. An extremely small part, quantity, or amount.

Related Forms:

• a·tomˈic (ə-tŏmˈĭk) adjective