What is a reflex action?

A reflex action is an automatic and immediate response to a stimulus that occurs without conscious thought or effort. 

The reflex arc consists of a sensory receptor that detects the stimulus, a sensory neuron that transmits the information to the spinal cord, an interneuron that processes the information and decides on the appropriate response, and a motor neuron that transmits the response to the muscles or glands that will carry out the action.