What is ICD-9 in medical terminology?

ICD-9 stands for International Classification of Diseases, 9th revision.  The popular version that is in use is ICD-9-CM.  ICD-9 is a set of codes used to communicate the diagnosis of a patient made by the doctor or a procedure ordered by a doctor.  The standard terminology is followed across the industry so that different players or parts, like the hospitals, labs, payers, providers, all talk the common knowledge and have a common definition of what a particular diagnosis means.