The planetarium is a facility to simulate the motions of the sky and the planets. A special projector displays the astronomical objects life-like on the arced ceiling of the dome. The visitors sit in comfortable, swivel-mounted armchairs and have an all-round sight. The performance equals a multi-media presentation: Using slides, video clips, sound effects, allsky images, and panoramas, the audience is directly involved in the astronomical occurences. The show is controlled by a computer software; though, it can also be held live by a lecturer.

The term planetarium is used for the whole facility that carries out the star show. Sometimes it is called "star theater". The main components are:

• the dome, a high semi-spherical ceiling on which the presentation takes place;
• the projector, the main device to display the starry sky and the astronomical motions; and
• a service area, the associated space for the technical instrumentation, offices, cafe, lecturing hall etc.

Originally, "planetarium" denoted a mechanical table-toy which reproduced the motions of the planets (see: History - Orrery). Since the development of the projection planetarium in the 1920ies, the term altered to its present meaning.