A reflex is an automatic muscle response by a muscle that is generally under voluntary control to sensory input without the intervention of the brain. Typical reflex responses are the contraction of the upper thigh muscle when the kneecap is stimulated, the spreading of the toes when the sole of the foot is touched, or the blink response when anything is heading towards the eye.
When a patient is suspected of having nerve damage, but not brain damage, doctors will often test reflexes to see if the response is typical.