The theory that physiological and emotional changes occur simultaneously in response to a stimulus (as opposed to the earlier James-Lange theory).
Example: A woman is hiking in the forest when she stumbles upon a bear. All at once, she starts sweating, trembling, and feeling extremely afraid.
I think this link explains the theory in the greatest detail, It has diagrams to illustrate the main idea such as this one: http://en.academic.ru/dic.nsf/enwiki/1716266
STIMULUS (Bear) –> EMOTION (Fear) –> REACTION/RESPONSE (Run Away)
Fournier, G. (2010). Cannon-Bard Theory. Psych Central. Retrieved on May 28, 2015, from http://psychcentral.com/encyclopedia/2008/cannon-bard-theory/