Pathognomonic

Pathognomonic (rare synonym pathognomic) is a term, often used in medicine, that means "characteristic for a particular disease". A pathognomonic sign is a particular sign whose presence means that a particular disease is present beyond any doubt. Labelling a sign or symptom "pathognomonic" represents a marked intensification of a "diagnostic" sign or symptom.

The word is an adjective of Greek origin derived from πάθος pathos "disease" and γνώμων gnomon "indicator" (from γιγνώσκω gignosko "I know, I recognize").