To assure is (1) to make sure something occurs, (2) to give confidence to or encourage, or (3) to make (someone) certain (of something). In its first sense at least, the word is synonymous with ensure, which primarily means to make sure something occurs or comes to be. Insure, meanwhile, usually means to purchase or provide financial protection against loss.
This is how the words are usually used in today’s English, anyway. Each has borne many other definitions over the centuries, and they have tended to share many of their senses. Today, though, we can think of assure as (roughly) synonymous with inform or promise, ensure as synonymous with make sure, and insure as mainly confined to uses relating to financial protection for loss.
In British English, assurance was once the preferred term for some types of insurance. The older form lives on in some company names, but it has largely given way to insurance.
And after some hands-on time with the new game, I can assure BioShock fans that they need not fear the change. [Mirror]
Good land, impressive yields, and federal tariff protection ensured profit margins for the antebellum elite. [Technology, Innovation, and Southern Industrialization, Susanna Delfino and Michele Gillespie]
As the artworks gained value over the years, however, so has the cost of insuring them. [CBC]
There are two ways to guarantee privacy: by assuring anonymity and by assuring confidentiality. [Mass Media Research, Roger D. Wimmer, Joseph R. Dominick]
Defense Secretary Leon Panetta said Friday that the U.S. is “working around the clock” to ensure the safe return of American hostages. [TPM]
The improving mood has been seen in plunging yields on Dubai bonds and in the credit default swaps used to insure against the possibility of the government defaulting. [BDLive]