a fold of transparent or semitransparent membrane, present in many vertebrates, that can be drawn laterally over the eye like a third eyelid, independently of the other eyelids. This “blink” (the nictitating-membrane response) can be conditioned in a nonhuman animal (e.g., a rabbit) by pairing a puff of air (the unconditioned stimulus) with a light or tone (the conditioned stimulus).