The Babinski sign is a reflex action observed in response to stimulation of the sole of the foot. When the outer edge of the foot is stroked, a normal response in adults is to curl the toes downward. However, in infants or individuals with certain neurological conditions, the big toe may extend upward while the other toes fan out. This sign is named after the French neurologist Joseph Babinski, who first described it in the late 19th century. The presence of the Babinski sign in adults can indicate damage to the central nervous system or other neurological disorders.