Although “ensure” and “insure” are often correctly used interchangeably, since the 17th century insure has been more specifically associated with the act of making something safe against loss through the payment of premiums. Ensure is more often used as a word meaning to protect or guarantee in a general sense.
Ensure is the older word, originating in the late 14th century from the Anglo-French “enseurer,” which translates as “make sure." Insure entered the language in the 15th century as a Middle English variant of ensure. Both words are related to “assure.” When insure took on its financial association in the 1600s, it replaced the then-common usage of assure.
The three words are synonyms but each carries a subtle distinction in meaning. To assure is to convince someone about something, as when a husband assures his wife that he intends to deposit his entire paycheck in the bank. Ensure means to guarantee something happening or not happening in the future. For instance, covering a stack of wood with plastic ensures that the wood stays dry when it rains.
In modern usage, insure usually refers to insurance, which provides financial compensation against loss or damage. The term also applies to payment secured for people to cover some or all of the costs of medical procedures or to provide monetary compensation for survivors of a deceased person.