To assure someone means to tell them something confidently and firmly to remove their doubts or worries. When your teacher assures you that the upcoming test will only cover material you've studied in class, she's promising you this is true so you won't worry unnecessarily. When a doctor assures a nervous patient that a procedure won't hurt, he's offering confident reassurance based on his knowledge and experience.

The word means speaking with enough certainty and sincerity to make someone feel better. You might assure your younger sibling that thunder can't hurt them, or a coach might assure the team that they're prepared for the championship game.

People often confuse assure with ensure and insure. Remember: you assure a person (give them confidence), you ensure that something happens (make certain of an outcome), and you insure property (protect it financially). So you might assure your parents that you'll be careful, ensure you follow all the safety rules, and insure your bike against theft.

When something is assured, it’s guaranteed or certain to happen: victory seemed assured once the team took a commanding lead.