1 [count] : an action or movement of the body that happens automatically as a reaction to something
the cough/gag reflex
reflexes such as swallowing and blinking
a reflex action/response
2 [count] : something that you do without thinking as a reaction to something
Disagreeing with my suggestions has become almost a reflex for him.
3 reflexes [plural] : the natural ability to react quickly
an athlete with great/quick reflexes
My reflexes are slower now that I'm older.