A sphere is a perfectly round, three-dimensional object where every point on its surface is the same distance from its center. Think of a basketball, a marble, or a soap bubble: each one is a sphere (or very close to it).

Spheres appear throughout nature and human design. Many planets are close to spherical because gravity pulls matter toward the center. Bubbles form spheres because that shape uses the least amount of surface area to contain a volume of air. Ancient Greek mathematicians studied spheres intensely, fascinated by their symmetry.

The word also refers to an area of activity, knowledge, or influence. Someone's sphere of influence is the range of people or situations they can affect. When you talk about the “political sphere” or the “sphere of education,” you mean that whole realm or domain of activity. A teacher's sphere of expertise might be science, while a coach's sphere might be athletics.

When describing something sphere-shaped, you can use the word spherical. Earth isn't a perfect sphere (it's slightly flattened at the poles), but we call it spherical because it's so close. Understanding spheres matters in everything from designing ball bearings that reduce friction to calculating how much wrapping paper you need to cover a round gift.