When you are asleep, sometimes you dream. Dreams are a little like a movie that plays in your head while you sleep. Sometimes dreams are very ordinary, sometimes they are exciting, and sometimes they are scary. Scary dreams are called nightmares. Often things happen in dreams that aren't possible in real life.
His forHallucination
A nightmare is a bad dream. Nightmares can be extremely frightening, and make you wake up scared.
You might also use nightmare to describe a particularly bad event, for example "we had a nightmare vacation".
A wish is something you really, really want. Some people wish for possible things, like "I wish I had a better job" but other people might wish for impossible things, like the man in the picture has wished he can fly.
Some things are said to be able to grant wishes. When you blow out your birthday candles you can make a wish, and fairies are said to be able to grant wishes.