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):

diagnosis
    n 1: identifying the nature or cause of some phenomenon [syn:
         diagnosis, diagnosing]

Moby Thesaurus II by Grady Ward, 1.0:

51 Moby Thesaurus words for "diagnosis":
   Pap test, action, anatomic diagnosis, award, biological diagnosis,
   biopsy, clinical diagnosis, condemnation, consideration,
   construction, cytodiagnosis, decision, decree, definition,
   deliverance, description, determination, dictum,
   differential diagnosis, digital examination, doom,
   electrocardiography, electroencephalography, electromyography,
   examination, finding, interpretation, laboratory diagnosis,
   mammography, meaning, oral examination, order, physical diagnosis,
   physical examination, postmortem diagnosis, precedent, prognosis,
   pronouncement, reading, resolution, ruling, sentence,
   serodiagnosis, smear, study, test, urinalysis, uroscopy, verdict,
   way of seeing, work-up



The Devil's Dictionary (1881-1906):

DIAGNOSIS, n.  A physician's forecast of the disease by the patient's
pulse and purse.