A mirror ball, disco ball, glitter ball, or ball mirror is a roughly spherical object that reflects light directed at it in many directions, producing a complex display. Its surface consists of hundreds or thousands of facets, nearly all of approximately the same shape and size, and each having a mirrored surface. Usually it is mounted well above the heads of the people present, suspended from a device that causes it to rotate 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 called "disco balls" were first used in nightclubs in the 1920s. _Wikipedia