The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   2. Hence, the act or process of identifying the nature or
      cause of some phenomenon, especially the abnormal behavior
      of an animal or artifactual device; as, diagnosis of a
      vibration in an automobile; diagnosis of the failure of a
      sales campaign; diagnosis of a computer malfunction.
      [PJC]

   3. Scientific determination of any kind; the concise
      description of characterization of a species.
      [1913 Webster]

   4. Critical perception or scrutiny; judgment based on such
      scrutiny; esp., perception of, or judgment concerning,
      motives and character.
      [1913 Webster]

            The quick eye for effects, the clear diagnosis of
            men's minds, and the love of epigram. --Compton
                                                  Reade.
      [1913 Webster]

            My diagnosis of his character proved correct. --J.
                                                  Payn.
      [1913 Webster]

   Differential diagnosis (Med.), the determination of the
      distinguishing characteristics as between two similar
      diseases or conditions.
      [1913 Webster]

WordNet (r) 3.0 (2006):

differential diagnosis
    n 1: a systematic method of diagnosing a disorder (e.g.,
         headache) that lacks unique symptoms or signs