As to legal usage: in Michigan, versions of the terms "insure" and "ensure" are sometimes used interchangeably. Generally, however, "insure/insurance/insured/insurer" pertain to commercially purchased protection against the risk of loss, while "ensure/ensurance/ensured/ensurer" pertain to guarantees, such as a third-party guaranty of payment under a contract if the principal does not fulfill his obligation, and "assure/assurance/assured/assurer" are sometimes used as alternatives of the "ensure" group. The distinction between an insurer and an ensurer can be really blurred when the contract ensurer is an insurer. I have never seen a case in which the insure/ensure/assure issue made a difference.