1 (noncount) : the act of identifying a disease, illness, or problem by examining someone or something
The unusual combination of symptoms made accurate diagnosis difficult.
She is an expert in the diagnosis and treatment of eye diseases.
2 (count) : a statement or conclusion that describes the reason for a disease, illness, or problem
The diagnosis was a mild concussion.
His doctor made an initial diagnosis of pneumonia.
The committee published its diagnosis of the problems affecting urban schools.