1. The quality or state of being irritable; testiness or petulance.
2. Pathology. Abnormal or excessive sensitivity of a body organ or part to a stimulus.
3. Physiology. The capacity to respond to stimuli.

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.