Certain means being completely sure about something, with no doubt in your mind. When you're certain the sky is blue, you know it as a fact. When a scientist is certain that water freezes at 32 degrees Fahrenheit, she has tested it so many times that there's no question left.

Being certain is different from thinking something is probably true. You might think your team will probably win the game, but you can't be certain until the final whistle blows. You might guess that your friend is home, but you're only certain after you call and hear her voice.

Sometimes people use certain to mean “some” or “particular,” as in “certain students” or “certain rules.” This means specific ones, though not necessarily all of them. A teacher might say that certain assignments are required, meaning particular ones she has in mind.

The word connects to certainty (the state of being sure) and certainly (without doubt). When someone asks if you're coming to the party and you reply “certainly,” you mean absolutely yes, with no question about it.