Source : Webster's Revised Unabridged Dictionary (1913)

Diagnosis \Di`ag*no"sis\, n.; pl. {Diagnoses}. [NL., fr. Gr. ?,
   fr. ? to distinguish; dia` through, asunder + ? to know. See
   {Know}.]
   1. (Med.) The art or act of recognizing the presence of
      disease from its signs or symptoms, and deciding as to its
      character; also, the decision arrived at.

   2. Scientific determination of any kind; the concise
      description of characterization of a species.

   3. Critical perception or scrutiny; judgment based on such
      scrutiny; esp., perception of, or judgment concerning,
      motives and character.

            The quick eye for effects, the clear diagnosis of
            men's minds, and the love of epigram. --Compton
                                                  Reade.

            My diagnosis of his character proved correct. --J.
                                                  Payn.

   {Differential diagnosis} (Med.), the determination of the
      distinguishing characteristics as between two similar
      diseases or conditions.

Source : WordNet®

differential diagnosis
     n : a systematic method of diagnosing a disorder (e.g.,
         headache) that lacks unique symptoms or signs