The word "certain" is an adjective that is used to describe something that is known for sure, having no doubt about it. It can also refer to something that is specific or particular. For example, if someone says, "I am certain that the meeting is at 3 PM," it means they have no doubt about the time of the meeting.
The word "certain" comes from the Old French word "certain," which is derived from the Latin word "certus," meaning "determined" or "fixed." The Latin root brings the idea of an assured or definite quality.