1. The ancient theory of Democritus, Epicurus, and Lucretius, according to which simple, minute, indivisible, and indestructible particles are the basic components of the entire universe.
2. A theory according to which social institutions, values, and processes arise solely from the acts and interests of individuals, who thus constitute the only true subject of analysis.

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.