A planetarium was a theater built primarily for presenting educational and entertaining shows about astronomy and the night sky.
In 1996, Griffith Observatory featured a planetarium show, which was hosted by Rain Robinson on Tuesday nights. (VOY: "Future's End")
When Mirasta Yale was nine years old, her parents took her to a planetarium on Malcor III. They would sit in the dark and watch a program, which caused Yale to become fascinated with the idea of traveling on a spaceship to meet people on other planets. (TNG: "First Contact")