Lucid dreaming

A regular, ordinary dream is a state in which you are vividly hallucinating while comatose. That's basically what it is. In a regular dream, you are not aware that you are dreaming and your hallucinations feel like reality.
As an interesting side note, while sleeping, and more specifically, dreaming in an REM state, your body is paralyzed - this is so that movements in the dream world don't carry over to your physical body in reality. However, this state can also be activated by your brain before or after you actually fall asleep/wake up. If you're waking up, this paralysis can last for seconds, minutes, and occasionally even hours. The person experiencing this sensation can move their eyes, as sleep paralysis doesn't affect the eyes, and often experiences vivid hallucinations, which can cause them to erroneously conclude that they are still dreaming, as they might see fantastical objects alongside ordinary ones in the space in which they were sleeping. It can be nightmarish and potentially terrifying if you hallucinate about scary things, because you will see, very vividly, scary things in your sleeping space, and on top of this, you can't move, only watch.
Anyway, back to talking about ordinary dreams. The key fact that I said up there is that in an ordinary dream, your hallucinations feel like reality.
There is a fascinating phenomenon called lucid dreaming. In a lucid dream, the dreamer is aware that the world they see (are dreaming about) is not reality. Basically, they know that they are in a dream.
Lucid dreamers can often manipulate the dream world, too - and because it's a dream, they can literally do anything. See from two perspectives, defy physics, and so much more. Lucid dreamers can also consciously talk to their subconscious, by which they could potentially find answers to extremely difficult questions that they are aware of subconsciously but cannot conclude consciously. Haha, I sound like I'm writing an ad for lucid dreaming.
But seriously, it is really cool. I think one of the most trippy things about it is that lucid dreaming is actually a completely separate level of consciousness. An EEG machine recorded brain wave activities in lucid dreamers, and came up with frequencies up to 40 Hz (Gamma). For comparison, regular dreamers show frequencies between 4-8 Hz (Theta) and conscious, awake people show frequencies between 12-38 Hz (Beta). Trippy.