From Greek 'diagnōsis' (discernment), combining 'dia-' (through/apart) and 'gignōskein' (to know) — literally 'knowing apart', the act of distinguishing one disease from another, rooted in PIE *ǵneh₃- which also gave us 'know' and 'cognition'.
The identification of the nature of an illness or other problem by examination of the symptoms.
From Greek 'diagnōsis' (διάγνωσις: discernment, discrimination, a deciding), formed from 'dia-' (through, apart, across) + 'gignōskein' (to know, to perceive, to recognize), from PIE *ǵneh₃- (to know). This PIE root is one of the most productive in the Indo-European family, giving Latin 'gnoscere/noscere' (to know), English 'know,' 'can,' 'cunning,' and 'ken,' as well as Greek 'gnōsis' (knowledge) and 'gnōmē' (judgment). The prefix 'dia-' sharpens the meaning: to know
The PIE root *ǵneh₃- ('to know') is one of the most prolific roots in the Indo-European family — it gives us Greek 'gnosis', Latin 'cognoscere' (→ cognition, recognize), Old English 'cnāwan' (→ know), and Sanskrit 'jñā' (→ jñāna, as in Jnana yoga). Diagnosis, knowledge, and know are all cousins.