A disco ball is a roughly spherical

Sphere

A sphere is a perfectly round geometrical object in three-dimensional space, such as the shape of a round ball. Like a circle in two dimensions, a perfect sphere is completely symmetrical around its center, with all points on the surface lying the same distance r from the center point...

 object that reflects light directed at it in many directions, producing a complex display. Its surface consists of hundreds or thousands of facet

Facet

Facets are flat faces on geometric shapes. The organization of naturally occurring facets was key to early developments in crystallography, since they reflect the underlying symmetry of the crystal structure...

s, nearly all of approximately the same shape and size, and each having a mirror

Mirror

A mirror is an object that reflects light or sound in a way that preserves much of its original quality prior to its contact with the mirror. Some mirrors also filter out some wavelengths, while preserving other wavelengths in the reflection...

ed surface. Usually it is mounted well above the heads of the people present, suspended from a device that causes it to rotate

Rotation

A rotation is a circular movement of an object around a center of rotation. A three-dimensional object rotates always around an imaginary line called a rotation axis. If the axis is within the body, and passes through its center of mass the body is said to rotate upon itself, or spin. A rotation...

 steadily on a vertical axis, and illuminated by spotlights, so that stationary viewers experience beams of light flashing over them, and see myriad spots of light spinning around the walls of the room.

What are now sometimes called "disco balls" were first widely used in nightclubs in the 1920s.