Babinski reflex: A reflex used to determine adequacy of the higher (central) nervous system. The Babinski reflex is obtained by stimulating the outside of the sole of the foot, causing extension of the big toe while fanning the other toes. The examiner begins the stimulation at the heel and goes forward to the base of the toes. Most newborn babies and young infants are not neurologically mature, and they therefore show a Babinski reflex. A Babinski reflex in an older child or an adult is abnormal and is a sign of a problem in the brain or spinal cord. A Babinski reflex that is present on one side but not the other is also abnormal, and it can indicate which side of the brain is involved. Also known as plantar response, big toe sign, and Babinski phenomenon, response, or sign.