Strictly speaking, it is incorrect to use insure and ensure interchangeably. Ensure means to make sure or to guarantee and insure refers to the financial service of insurance.
|Meaning||to make sure, to guarantee||to safeguard against loss or damage (by buying insurance).|
The word insure is used mostly in a financial context as it relates to insurance. For example,
- It is required by law that you insure your car.
- I use the same company to insure both my home and my car.
- The painting was stolen but the owner was not worried because it was insured. In fact, his wife was glad because she found it pretty ugly.
The word ensure usually means "to make sure" or "to guarantee". For example,
- Leaving your car unlocked in this neighborhood will ensure your stereo gets stolen.
- Sequestering the jury will ensure that the defendant gets a fair trial.
- Religious indoctrination of impressionable young minds ensures they lose the ability to think critically and independently.
edit Video explaining the ensure - insure difference
edit Usage in the U.S.
In colloquial usage — especially in the United States — the words insure and ensure are indeed used interchangeably. So some dictionaries list a secondary meaning of the word insure as a synonym for ensure.
"Ensure vs Insure." Diffen.com. Diffen LLC, n.d. Web. 28 Nov 2014. < http://www.diffen.com/difference/Ensure_vs_Insure >