In medicine, a sign is an objective observation, on physical examination or other test, that indicates a specific abnormality. They complement symptoms, the subjective description by the patient of his own experience. Symptoms come from history-taking while signs come from physical examination, imaging, blood analysis, etc. Both symptoms and signs go into a diagnosis.
"I have the worst headache of my life" is a symptom, and one that a competent professional doing triage would consider an emergency, at least until the blood pressure was taken. "Blood pressure is 240/180" is a sign, and an extremely critical one.
An abnormal response on physical examination, such as the failure of the pupils to contract when a light is shined into the eye, would be a sign on physical examination.
Signs often carry "eponyms", or the name of the clinician that first described them. For example, Tinel's sign, suggestive of a nerve entrapment, results when the patient reports tingling or pain when the path of the nerve is gently tapped.