Objective indication of some medical fact or characteristic that may be detected by a physician during a physical examination or by a clinical scientist by means of an in vivo examination of a patient.
For example, whereas paresthesia is a symptom (only the person experiencing it can directly observe their own tingling feeling), erythema is a sign (anyone can confirm that the skin is redder than usual). Symptoms and signs are often nonspecific, but often combinations of them are at least suggestive of certain diagnoses, helping to narrow down what may be wrong. In other cases they are specific even to the point of being pathognomonic.
Some signs may have no meaning to the patient, and may even go unnoticed, but may be meaningful and significant to the healthcare provider in assisting diagnosis.
Examples of signs include elevated blood pressure, a clubbing of the fingers (which may be a sign of lung disease, or many other things), and arcus senilis.
The term sign is not to be confused with the term indication, which denotes a valid reason for using some treatment.