Holodeck
Also known as the Holographic Environment Simulator. A Holodeck is a large chamber whose walls are made up of billions of microscopic "holoprojectors". These holoprojectors run in accordance with an overall holodeck program to create an environment. Each projector creates a small piece of matter in the chamber. This matter ("holo-matter") is simply a hologram inside a force field, shaped so perfectly that it is indistinguishable from a real object. Holo-matter can not exist outside the holodeck.
When all the billions of microscopic holo-projectors work in concert, the overall effect is a flawless simulation of any environment imaginable.
Holodecks are kept on board starships as places for recreation. Users can visit their childhood homes, their favourite bar, a wildlife park in the middle of Africa, a theme park or anything else you could possibly imagine - all without leaving that one chamber. Users can even play the role of the hero in their favourite movie or novel.
Smaller versions of Holdecks called Holosuites are kept on Deep Space Nine.