Entry 2 senses · 2 variants Webster, 1913 Diagnostic /dīˌ-əg-näs'-tĭk/ · Di·ag·nos·tic · IPA /ˌdaɪ.əɡˈnɑ.stɪk/ a. n. 01 a. Pertaining to, or furnishing, a diagnosis; indicating the nature of a disease. 1. Pertaining to, or furnishing, a diagnosis; indicating the nature of a disease. 02 n. The mark or symptom by which one disease is known or distinguished from others. 1. The mark or symptom by which one disease is known or distinguished from others.