The earliest documented case of a disease. This is usually used in reference to a particular outbreak of a disease, or the first documented case of a disease in a specific population.
This is largely a synonym for Patient Zero, but patient zero is usually used in the specific case of Gaetan Dugas and AIDS. Also, the index case might have died before being diagnosed, making the term 'patient' inappropriate. For whatever reason, index case is the term most likely to be used by epidemiologists.