Do you ensure or insure?

There is considerable confusion about whether ensure and insure are distinct words, variants of the same word, or some combination of the two. They are in fact different words, but with sufficient overlap in meaning and form as to create uncertainty as to which should be used when. We define ensure as “to make sure, certain, or safe” and one sense of insure, “to make certain especially by taking necessary measures and precautions,” is quite similar. But insure has the additional meaning “to provide or obtain insurance on or for,” which is not shared by ensure. Some usage guides recommend using insure in financial contexts (as in “she insured her book collection for a million dollars”) and ensure in the general sense “to make certain” (as in “she ensured that the book collection was packed well”).