Sure means certain or confident about something. When you're sure you locked the door, you have no doubt about it. When a scientist is sure about her findings, she has checked them carefully and believes they're correct. The opposite of being sure is being uncertain or doubtful.

The word also means reliable or steady. A sure-footed mountain goat can climb steep cliffs without slipping. A surefire way to make friends is a method that works very well. When you want something done right, you look for someone with a sure hand, someone whose skills you can count on.

People also use “sure” as a casual way to say yes. If your friend asks whether you want to play soccer after school, you might answer “Sure!” This relaxed use of the word suggests easy agreement, like saying “yes, definitely” without making a big deal about it.

The phrase to be sure means “admittedly” or “certainly,” as in “The hike was difficult, to be sure, but the view from the summit made it worthwhile.” And when someone says sure enough, they mean something happened exactly as expected: “I thought it might rain, and sure enough, it started pouring an hour later.”