Pathology: The study of disease. Pathology has been defined as "that branch of medicine which treats of the essential nature of disease." The word "pathology" comes from the Greek words "pathos" meaning "disease" and "logos" meaning "a treatise" = a treatise of disease. The word "pathology" is sometimes misused to mean disease as, for example, "he didn't find any pathology" (meaning he found no evidence of disease). A medical doctor that specializes in pathology is called a pathologist. Pathologists are experts at interpreting microscopic views of body tissues.