A lucid dream was a technique allowing one to take control of ones dream. They could do anything that they want as long as they stay in control, even waking up whenever they want.
Tom Paris once had an experience in which after realizing he was dreaming he was able to fly. In 2374, Chakotay used a lucid dream to make contact with a group of unnamed aliens that were taking over the crew of USS Voyager in their dreams, choosing the image of Earth's Moon to "remind" him that he was dreaming. (VOY: "Waking Moments")