Diagnostic

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early 17th century: from Greek diagnōstikos ‘able to distinguish’, from diagignōskein ‘distinguish’; the noun from hē diagnōstikē tekhnē ‘the art of distinguishing (disease)’.

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etymonline

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diagnostic (adj.)

1620s, "of or pertaining to diagnosis," also as a noun, "a symptom of value in diagnosis," from Greek diagnōstikos "able to distinguish," from diagnōstos, verbal adjective from diagignōskein "to discern, distinguish," literally "to know thoroughly" or "know apart (from another)," from dia "between" (see dia-) + gignōskein "to learn, to come to know," from PIE root *gno- "to know." Related: Diagnostics.