—adj. 1. free from doubt as to the reliability, character, action, etc., of something: to be sure of one's data. 2. confident, as of something expected: sure of success. 3. convinced, fully persuaded, or positive: to be sure of a person's guilt. 4. assured or certain beyond question: a sure victory. 5. worthy of confidence; reliable; stable: a sure messenger. 6. unfailing; never disappointing expectations: a sure cure. 7. unerring; never missing, slipping, etc.: a sure aim. 8. admitting of no doubt or question: sure proof. 9. destined; bound inevitably; certain: sure death. 10. Obs.secure; safe.
11. be sure, to take care (to be or do as specified); be certain: Be sure to close the windows. 12. for sure, as a certainty; surely: It's going to be a good day, for sure. 13. make sure, to be or become absolutely certain: I'm calling to make sure that you remember to come. 14. sure enough,Informal.as might have been supposed; actually; certainly: Sure enough, the picnic was rained out. 15. to be sure, a. without doubt; surely; certainly.
b. admittedly: She sings well, to be sure, but she can't act.
—adv. Informal.certainly; surely: It sure is cold out. Sure, I'll come.