di·ag·nos·tic 
(dī′əg-nŏstĭk)
(dī′əg-nŏstĭk)adj.
1. Of, relating to, or used in a diagnosis.
2. Serving to identify a particular disease; characteristic.
n.
1. often diagnostics (used with a sing. verb) The art or practice of medical diagnosis.
2. A symptom or a distinguishing feature serving as supporting evidence in a diagnosis.
3. An instrument or technique used in medical diagnosis.
[Greek diagnōstikos, able to distinguish, from diagnōstos, distinguished, from diagignōskein, to distinguish; see DIAGNOSIS.]
di′ag·nosti·cal·ly adv.