A planetarium is a facility where people watch sky shows projected onto a large, usually domed, ceiling. Sky shows are simulations of astronomical views, objects, and events. Planetariums often include additional exhibits on astronomy and/or spaceflight.

The audience watches a simulation projected inside the facility. This differs from an observatory, where instruments observe the actual sky and outer space.

Astronomy or spaceflight exhibits can accompany the sky-show auditorium. A domed ceiling is usual, but the projected astronomical presentation is the central activity.