A semicircle is exactly half of a circle, created when you slice a circle perfectly through its center. If you drew a circle on paper and folded it in half, the curved edge you see is a semicircle.

You see semicircles everywhere: a rainbow can look like a semicircle across the sky, and many doorways have semicircular arches at the top. Ancient Roman architects used semicircular arches because they help distribute weight down both sides.

In math class, you might calculate the perimeter of a semicircle by finding half the circle's circumference and adding the diameter (the straight edge across). The area equals half of what the full circle's area would be.

Semicircles appear in sports too: the three-point line in basketball forms a semicircle around each basket, and the key (the painted area under the hoop) often includes a semicircular section. When you're playing defense and guarding the goal in soccer or hockey, you might arrange your teammates in a semicircle to cover more ground.