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diagnostic (adj.)

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