diagnostic

Concise Oxford English Dictionary © 2008 Oxford University Press:
diagnostic /dʌɪəgˈnɒstɪk/

▶adjective

• 1 concerned with the diagnosis of illness or other problems.

  ■ (of a symptom) distinctive, and so indicating the nature of an illness.

• 2 characteristic of a particular species, genus, or phenomenon.

▶noun

• 1 a distinctive symptom or characteristic.

• 2 (diagnostics) [treated as sing. or pl.] the practice or techniques of diagnosis.

• 3 Computing a program or routine that helps a user to identify errors.

– derivatives
diagnostically adverb,
diagnostician /-nɒˈstɪʃ(ə)n/ noun.