Certain (adjective)

1. Definitely known or established.
2. Fully confident or sure of something.
3. Certain to happen or to be the case.
4. Excluding no one or nothing; all.
5. Determined or fixed, but not specified.

Origin:

From old french certen, from latin certus "fixed, settled, sure".

Examples:

1. I"m certain that he"s coming.
2. It's certain that the weather will be bad tomorrow.
3. I'm certain of my answer.
4. I"m certain that I"ve met you before.
5. I'm certain that I locked the door.