For certain Definition 1 of 2Pro Tip 1/3Certainty Level Used to show you are completely sure about your knowledge or belief. I can't say for certain if it'll rain tomorrow. Pro Tip 2/3Stress Importance Emphasize the absence of doubt in what is being stated or predicted. The test results will come out negative for certain. Pro Tip 3/3No Need for Proof Express strong conviction even when proof is unavailable. She said for certain that she saw a dolphin in the river.