An automatic, unthinking, or reflexive response, usually to a situation or stimulus.

स्वतःस्फूर्त, बिना सोचे-समझे प्रतिक्रिया

English Usage: His knee jerk reflex to the criticism was to get defensive.

Hindi Usage: आलोचना पर उसकी घुटना-झटका प्रतिवर्त तुरंत रक्षात्मक हो गई।