1. A great or plentiful amount.
2. Fullness to overflowing: "My thoughts . . . are from the abundance of my heart" (Thomas De Quincey).
3. Affluence; wealth.
4. Chemistry. The amount of an isotope of an element that exists in nature, usually expressed as a percentage of the total amount of all isotopes of the element.

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.