An action performed independently of the will, as an automatic response to a sensory stimulus; (more widely) an action that is automatic, unthinking, or instinctive; also in extended use and figurative.
Origin
Mid 19th century.
Definition of reflex action in US English:
reflex action
noun
An action performed independently of the will, as an automatic response to a sensory stimulus; (more widely) an action that is automatic, unthinking, or instinctive; also in extended use and figurative.