Original: The International Statistical Classification of Diseases and Related Health Problems (most commonly known by the abbreviation ICD) provides codes to classify diseases and a wide variety of signs, symptoms, abnormal findings,  complaints, social circumstances and external causes of injury or disease.
Simplified: The International Classification of Diseases and Related Health Problems (also known as ICD) is a list of codes to classify diseases, including signs, symptoms, abnormal findings, social circumstances, complaints and external causes of injury or disease.