1. Three-digit code that represents a single disease or a group of closely related conditions in the diagnostic code book titled International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM) . 2. Letter with two digits that represents a single disease or a group of closely related conditions in the diagnostic code book titled International Classification of Diseases, Tenth Revision, Clinical Modification (ICD-10-CM). Also referred to as three-character category.