To determine which disease is causing a sick person's signs and symptoms; to find the diagnosis.
2
(by extension) To determine the cause of a problem.
noun
1
The identification of the nature and cause of an illness.
He was given the wrong treatment due to an erroneous diagnosis.
2
The identification of the nature and cause of something (of any nature).
3
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.