A diameter is a straight line that passes through the center of a circle and touches both sides. If you drew a line from one edge of a pizza straight through the middle to the opposite edge, you'd have drawn a diameter. The diameter is always the longest line you can draw across a circle.

The diameter is twice as long as the radius, which is the distance from the center to the edge. So if a bicycle wheel has a radius of one foot, its diameter is two feet. Knowing the diameter helps you figure out other important measurements, like how far around the circle goes (its circumference).

You'll encounter diameter when measuring round objects: plates, wheels, pipes, or even planets. Scientists describe Earth's diameter as about 8,000 miles, meaning if you could travel straight through the center to the other side, you'd go about 8,000 miles.