Sudden & automatic response of part or whole of the body to a specific stimulus is called reflex action.
It was first discovered by Marshall Hall (1833)
It is an involuntary action in response to Stimulus & behaviour is inborn or stereotype.
The reflex action comprises reception, conduction, modulation & effects.
As for example:-
First of all an external stimulus is received by sensory nerves & converted into nerve impulse.
Such nerve impulse is conducted by sensory nerve to the brain and spinal cord, where impulse is interpreted & sends to the effector organ through motor nerves.
The nervous Pathway followed during reflex action is called reflex arc.
Some of the more examples are sneezing, hunger, Salivation, Cough reflex, Eye blinking etc.