Source : Webster's Revised Unabridged Dictionary (1913)

Diagnostic \Di`ag*nos"tic\, a. [Gr. ? able to distinguish, fr.
   ?: cf. F. diagnostique.]
   Pertaining to, or furnishing, a diagnosis; indicating the
   nature of a disease.

Diagnostic \Di`ag*nos"tic\, n.
   The mark or symptom by which one disease is known or
   distinguished from others.

Source : WordNet®

diagnostic
     adj 1: concerned with diagnosis; used for furthering diagnosis; "a
            diagnostic clinic"; "a diagnostic reading test";
            "diagnostic information"
     2: characteristic or indicative of e.g. a disease; "a
        diagnostic sign of yellow fever"; "diagnostic
        information"; "a rash symptomatic of scarlet fever";
        "symptomatic of insanity"; "a rise in crime symptomatic of
        social breakdown" [syn: {symptomatic}, {symptomatic of(p)}]