A back-formation from 'diagnosis,' from Greek 'diagignoskein' (to know apart) — distinguishing one disease from another.
To identify the nature of an illness or other problem by examination of the symptoms.
A back-formation from 'diagnosis,' which entered English via Latin from Greek 'diágnōsis' (διάγνωσις, a distinguishing, a discernment, a judicial decision, a determination of what a thing is), from 'diagignṓskein' (to distinguish thoroughly, to know one thing from another by careful examination), composed of 'dia-' (through, apart, across — carrying the sense of discrimination and penetration) + 'gignṓskein' (to know, to perceive, to recognise, to decide). The root of 'gignṓskein' is Proto-Indo-European *ǵneh₃- (to know), reconstructed as one of the most universally distributed roots in the family. Its reflexes span from India to Ireland: Sanskrit 'jñānam' (ज्ञान, knowledge — root of 'jnana' in yogic philosophy, the path of wisdom), Avestan 'zan' (to know), Greek 'gnṓmē' (judgment, opinion → 'gnome' as a wise saying), 'gnōsis' (γνῶσις, knowledge → Gnosticism and the Gnostic traditions), Latin 'gnōscere' and 'nōscere' (to know → 'cognition,' 'notion,' 'noble' — originally 'the known, the recognised person of
The word 'diagnose' literally means 'to know apart' — to distinguish one illness from another. It shares its root with 'know' (from Old English 'cnāwan') and 'gnosis' (Greek for knowledge). So a diagnosis is, at root, simply 'knowing through' — knowledge achieved by careful distinction.