adjective.
Specific to a certain disease or condition, as a symptom or finding on physical examination.
The Century Dictionary and Cyclopedia
In medicine, indicating that by which a disease may be certainly known; hence, belonging to or inseparable from a disease, being found in it and in no other; characteristic: as, pathognomonic symptoms.
the GNU version of the Collaborative International Dictionary of English
adjective.
Specially or decisively characteristic of a disease; indicating with certainty a disease.