A reflex action simply, is a response to a stimulus acting over a reflex arc. In a more explanatory sense, it is an automatic or involuntary response of the body to a given stimulus which involves the brain or spinal cord. If only the spinal cord is involved, it’s called spinal reflex, e.g knee jark, while if it involves the brain, it’s called cranial reflex, e.g contraction of the pupil in the presence of high light intensity. Reflex action occurs rapidly in a stereotyped manner i.e the response to a particular stimulus is the same at all times. They are often not under the control of the will. Hence, reflex actions are otherwise called involuntary action.
Examples of reflex actions are the quick withdrawal of the foot from a sharp object, blinking of the eyes as a small fly approaches it, jerking of the knee, sneezing when a foreign object approaches the nostril, contraction of the pupil when a bright light is shine into it.