1. 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.
2. An extremely small part, quantity, or amount.
3. Physics & Chemistry.
   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.

[Middle English attome, from Latin atomus, from Greek atomos, indivisible, atom : a-, not; see a-¹ + tomos, cutting (from temnein, to cut).]

The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.