A doctor's decision about the clinical state of the patient.

The primary diagnosis is the main condition for which health care is required. The main condition should be the underlying condition from which the patient suffers, even if this is not the immediately presenting problem. Every patient must have one primary diagnosis.
A secondary diagnosis is any additional condition, complication, or health problem from which the patient suffers. This may or may not be directly connected to the primary diagnosis.

The National Council for Palliative care. MSD Data Manual. 2009.