1. Extreme incipient inflammatory reaction of the tissues to an injury.
2. The normal response of nerve or muscle to a stimulus.
3. The evocation of a normal or exaggerated reaction in the tissues by the application of a stimulus.
The definition information for irritation is provided by Stedman's. You can search our medical dictionary here.
Stedman's, part of Lippincott Williams & Wilkins, provides a comprehensive line of health-science publications for healthcare professionals and medical students.