A/one hundred per cent Definition 1 of 3Pro Tip 1/3Expressing Certainty Use it when you're completely sure about something without any doubt. She passed her driving test with one hundred per cent certainty. Pro Tip 2/3Strong Agreement Shows total agreement with someone else's opinion or statement. Yes, I'm one hundred per cent in agreement with your plan. Pro Tip 3/3Emotional Emphasis Adds emotional weight, showing enthusiasm or strong feelings on a subject. He's one hundred per cent excited about the upcoming trip.