The definition of a sure thing is something definite, guaranteed or certain to happen.
An example of a sure thing is that a person will have to pay interest on credit card debt if he carries a balance and the card charges interest.
An outcome that is certain or assured; a certainty.
Of course; certainly.
Sure thing, I'll be there!
Something considered certain to win, succeed, etc.
All right; OK.
(informal) A reply to thank you.
"Thanks a lot for your help." / "Sure thing!"
(US) That which is certain.