Physically secure and certain, non-failing, reliable. / Certain in one's knowledge or belief. / (followed by a to infinitive) Certain to act or be a specified way.

(modal adverb) Without doubt, certainly. / (archaic) Without fail, surely.

Yes, expressing noncommittal agreement or consent. / Yes; of course. / You're welcome; polite response to being thanked.

Without fail. / Certainly, undoubtedly. / With confidence.

definitely, positively, without doubt

(idiomatic) Certainly, undoubtedly, admittedly.

(uncountable) The property of being sure, certainty. / (uncountable) The property of being sure, deft, confident of ability. / (countable, rare) The result or product of being sure.

plural of sureness

guaranteed to work

To verify, to recheck (with extra care or caution). / To ensure, to be sure to (to take care that something happens). / (followed by of) To check the reliability (of something).