1. 1. The act or process of expanding: the new nation's expansion westward.
   2. The state of being expanded.
2. 1. An expanded part: an expansion of a river.
   2. A product of expanding: a book that is an expansion of the author's Ph.D. thesis.
3. The extent or amount by which something has expanded.
4. Mathematics.
   1. A quantity written in an extended form, such as in a sum or product of terms.
   2. The process of obtaining this form.
5. An expanse.
6. A period of increased economic or business activity.

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.