unhealthy condition; poor health; indisposition; sickness.
Obsolete, wickedness.

1490–1500; ill + -ness

1. ailment, affliction, infirmity. Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
illness (ˈɪlnɪs)
1.  a disease or indisposition; sickness
2.  a state of ill health
3.  obsolete wickedness

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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American Heritage
Medical Dictionary

illness ill·ness (ĭl'nĭs)
Disease of body or mind; poor health; sickness.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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Encyclopedia Britannica


a harmful deviation from the normal structural or functional state of an organism. A diseased organism commonly exhibits signs or symptoms indicative of its abnormal state. Thus, the normal condition of an organism must be understood in order to recognize the hallmarks of disease. Nevertheless, a sharp demarcation between disease and health is not always apparent

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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Example sentences
Yet even when the disease is caused by the same mutation, it can provoke dramatically different degrees of illness.
Across the country, people with severe mental illness live what appear to be
  normal, successful lives.
Advanced dementia has often been treated as an amalgamation of symptoms in the
  aging, rather than a deadly illness in itself.
The evidence is mounting: mental illness might be caused by microbes.
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