The Collaborative International Dictionary of English v.0.48:

Pathognomonic \Pa*thog`no*mon"ic\, a. [Gr. ? skilled in judging
   of diseases; pa`qos a disease + ? skilled: cf. F.
   pathognomonique. See Gnomic.] (Med.)
   Specially or decisively characteristic of a disease;
   indicating with certainty a disease; as, a pathognomonic
   symptom.
   [1913 Webster]

         The true pathognomonic sign of love jealousy.
                                                  --Arbuthnot.
   [1913 Webster]