1. The act or process of reinforcing or the state of being reinforced.
2. Something that reinforces.
3. Additional personnel or equipment sent to support a military action. Often used in the plural.
4. Psychology.
   1. The occurrence or experimental introduction of an unconditioned stimulus along with a conditioned stimulus.
   2. The strengthening of a conditioned response by such means.
   3. An event, a circumstance, or a condition that increases the likelihood that a given response will recur in a situation like that in which the reinforcing condition originally occurred.

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.