From Stedman’s Medical Dictionary:

1. An interruption, cessation, or disorder of body functions, systems, or organs.illness, morbus, sickness;

2. A morbid entity characterized usually by at least two of these criteria:
- Recognized etiologic agent(s),
- Identifiable group of signs and symptoms
- Consistent anatomical alterations.
  - (See also syndrome.)

3. Literally, dis-ease, the opposite of ease, when something is wrong with a bodily function.