Dictionary

The identification of the nature and cause of an illness.

Example: He was given the wrong treatment due to an erroneous diagnosis.

The identification of the nature and cause of something (of any nature).

A written description of a species or other taxon serving to distinguish that species from all others. Especially, a description written in Latin and published.

To determine which disease is causing a sick person's signs and symptoms; to find the diagnosis.

(by extension) To determine the cause of a problem.