1. A physical or mental defect at the level of a body system or organ. The official WHO definition is: any loss or abnormality of psychological, physiologic, or anatomic structure or function.
The definition information for impairment 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.