The branch of medicine concerned with the study of the nature of disease and its causes, processes, development, and consequences. (wiktionary.com)
2. Any deviation from a healthy or normal condition; abnormality. (wiktionary.com)
3. The precise study and diagnosis of disease. (wikipedia.com)
4. A specialty concerned with the nature and cause of disease as expressed by changes in cellular or tissue structure and function caused by the disease process. (online-medical-dictionary.org)
Word origin: from Ancient Greek πάθος, pathos, feeling, suffering; and -λογία, -logia, the study of.