A dimension is a direction in which you can measure or move. The world around you has three dimensions: length (how long something is), width (how wide it is), and height (how tall it is). When you describe a box, you use all three: maybe it's 10 inches long, 6 inches wide, and 4 inches tall.

Think about drawing. A line on paper is one-dimensional because you can only measure its length. A rectangle is two-dimensional because it has length and width. But a real box sitting on your desk is three-dimensional because you can also measure how tall it is.

Scientists sometimes talk about time as a fourth dimension because events happen not just in space but also at particular moments. When you tell someone where to meet you, you need all four dimensions: the place (which requires three dimensions to locate exactly) and the time.

The word also means the size or extent of something: “We didn't realize the dimension of the problem until we saw how many people it affected.” In science fiction stories, writers imagine other dimensions as parallel worlds existing alongside our own, though this is different from the mathematical meaning.