Symptom

Symptoms are used in diagnosis. The term is used in medicine, and in other fields such as fault-finding in engineering. 

In medicine, it refers to things the patient feels or notices. It contrasts with signs, which are changes which a doctor sees in a patient. What a patient feels is subjective, whereas what a doctor sees is objective.

In diagnosis, all symptoms and signs are brought together to make a hypothesis as to what is going wrong. This might suggest further investigation or treatment. A complete investigation ends with a diagnosis that summarizes all this.

 Pain is a symptom, and red color is a sign, of a skin burn.
 Feeling sick is a symptom, and vomiting and diarrhea are signs, of food poisoning, cholera and other causes.
 Pain in the chest is a symptom, and cyanosis (blue color of skin) is a sign, of heart disease.
 Headaches, aching muscles, and nausea are symptoms of influenza.

 Similar terms 
Symptomatic: having or showing symptoms.
Asymptomatic: not showing symptoms.
Asymptomatic carrier has the disease but shows no symptoms.