Definition of "atom" []

• A part or particle considered to be an irreducible constituent of a specified system. (noun)
• The irreducible, indestructible material unit postulated by ancient atomism. (noun)
• An extremely small part, quantity, or amount. (noun)
• Physics & Chemistry 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. (noun)
• Physics & Chemistry This unit regarded as a source of nuclear energy. See Table at subatomic particle. (noun)

American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

• "We now suppose that it is _a rapid movement of electrons from atom to atom_ in the wire or wherever the current is."
• "Coined in ancient Greece, the term atom means “indivisible unit,” and through the nineteenth century, scientists believed that our entire physical universe was composed of these elementary particles."
• "While the term atom, therefore, is applicable only to elements, the term molecule is applicable both to elements and compounds."