Source : Webster's Revised Unabridged Dictionary (1913)

Illness \Ill"ness\, n. [From {Ill}.]
   1. The condition of being ill, evil, or bad; badness;
      unfavorableness. [Obs.] ``The illness of the weather.''
      --Locke.

   2. Disease; indisposition; malady; disorder of health;
      sickness; as, a short or a severe illness.

   3. Wrong moral conduct; wickedness. --Shak.

   Syn: Malady; disease; indisposition; ailment.

   Usage: {Illness}, {Sickness}. Within the present century,
          there has been a tendency in England to use illness in
          the sense of a continuous disease, disorder of health,
          or sickness, and to confine sickness more especially
          to a sense of nausea, or ``sickness of the stomach.''

Source : WordNet®

illness
     n : impairment of normal physiological function affecting part
         or all of an organism [syn: {unwellness}, {malady}, {sickness}]
         [ant: {health}, {health}]