The state of being unknown; a thing that is unknown.
The quality of being difficult to understand; a thing that is difficult to understand.
Darkness; the absence of light.
A signal; anything that indicates, or is characteristic of, the presence of something else, especially of something undesirable.
A perceived change in some function, sensation or appearance of a person that indicates a disease or disorder, such as fever, headache or rash; strictly, a symptom is felt or experienced by the patient, while a sign can be detected by an observer.