A sphere is a solid geometric figure defined as the set of all points equidistant from a common center point. It is perfectly symmetrical, and its surface is uniformly curved, without any edges, vertices or faces. It is the shape made when a semi-circle is rotated around its diameter.

A sphere has the smallest surface area able to contain a given volume. Soap bubbles tend to form spheres because the soap film is naturally drawn into the shape with the least amount of surface area in which to enclose the volume of air. Metaphorically, the word is often used to describe the place where a person or thing exists, as in the expression, "sphere of influence." This usage suggests that the person's influence is bounded or limited, just as the interior of a sphere is contained within its surface.