assure* v. to make certain or to state confidently.
ensure* v. to take steps so that something will happen.
insure* v. to accept payment of premiums against an unlikely, but foreseeable financial damage.
Words in the Wild: I can assure you that insuring your belongings will ensure your peace of mind.
If you follow the above links to the definitions at www.merriam-webster.com you’ll see that some dictionaries show that insure and ensure are interchangeable, and some sources also show that ensure and assure are considered to be synonymous in popular use.
I often edit work in which a writer uses insure in cases where ensure would, strictly speaking, be more accurate, so I know that this habit is in popular use. That said, maintaining each of these words’ distinct definitions allows writers and speakers to be more specific and convey their intended meaning more clearly. That’s why I, and many other finicky people, prefer to use these terms as defined above.
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