Is it ensure, insure, or assure?
early 15c., "to bring or put close," from approximate (adj.). Meaning "to come close" is from 1789. Related: Approximated; approximating.
approximate ap·prox·i·mate (ə-prŏk'sə-māt')
v. ap·prox·i·mat·ed, ap·prox·i·mat·ing, ap·prox·i·mates
To bring together, as cut edges of tissue. adj. (-mĭt)
Relating to the contact surfaces, either proximal or distal, of two adjacent teeth; proximate.
Close together. Used of the teeth in the human jaw.