>Rest assured you’re not the first person to be tripped up by insure, ensure, and assure. And for good reason -- there is a fair amount of leeway in the usage of these words.
But while a dictionary may state that it’s okay to use insure in a context other than insurance, and assure to mean “guarantee,” many style guides have stricter guidelines.
AP style, for instance, assigns one meaning to each word:
“Use ensure to mean guarantee . . . insure for references to insurance . . . assure to mean to make sure or give confidence.”
This approach simplifies the matter and makes the definitions easier to memorize. While you could defend your usage by pointing to Merriam-Webster, style guides often trump dictionaries.