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.
An indication that someone or something is in danger or at risk
A sign that indicates a danger, for example to vehicles on a road or of hazardous chemicals.