Little Nemo: Adventures in Slumberland Concepts

An alternate reality is a universe that is different from the main world of a series. This world can be different in just small ways and it will still resemble the main reality, or it can be completely different and not be similar at all to the universe the protagonist had known.

The imaginary events that goes on in your head when you are asleep. In some anime's they can bring important messages or to haunt a person.