The Collaborative International Dictionary of English v.0.48:

Hypochondriasis \Hy`po*chon"dri*a*sis\, n. [NL. So named because
   supposed to have its seat in the hypochondriac regions. See
   Hypochondriac, Hypochondrium, and cf. Hyp, 1st Hypo.]
   (Med.)
   A mental disorder in which melancholy and gloomy views
   torment the affected person, particularly concerning his own
   health; a morbid and deluded belief that one is afflicted
   with disease.

   Syn: hypochondria.
        [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Hypochondria \Hy`po*chon"dri*a\, n. [NL.] (Med.)
   An excessive concern about one's own health, particularly a
   morbid worry about illnesses which a person imagines are
   affecting him, often focusing on specific symptoms; also
   called hypochondriasis.
   [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

hypochondriasis
    n 1: chronic and abnormal anxiety about imaginary symptoms and
         ailments [syn: hypochondria, hypochondriasis]

The Devil's Dictionary (1881-1906):

HYPOCHONDRIASIS, n.  Depression of one's own spirits.

    Some heaps of trash upon a vacant lot
    Where long the village rubbish had been shot
    Displayed a sign among the stuff and stumps --
    "Hypochondriasis."  It meant The Dumps.
                                                        Bogul S. Purvy