Is it ensure, insure, or assure?
c.1500, "reflection of light," from verb reflex meaning "refract, deflect" (late 14c.), from Late Latin reflexus "a bending back," noun use of past participle of reflectere (see reflection). Meaning "involuntary nerve stimulation" first recorded 1877, from reflex action (1833).
reflex re·flex (rē'flěks')
An involuntary physiological response to a stimulus.
An unlearned or instinctive response to a stimulus.
Something, such as light or heat, that is reflected.
Being an involuntary action or response, such as a sneeze, blink, or hiccup.
Bent, turned, or thrown back; reflected.
To cause to undergo a reflex process.
An action or movement not controlled by conscious thought. A reflex may be anything from a hiccup to the involuntary response of a body part, such as the action that occurs in the knee-jerk reflex.