1) A mental state where one can not tell the difference between reality and their imagination.
3) A type of behaviour where a person may act in a way that may be seen as "uncool" or "weird" (and doesn't care).
3) The only way to fly.
Austin Powers: You're insane!
Goldmember: And that's the way, uh-huh, uh-huh, I like it!
A "character" created by the imagination to keep a person company. Sometimes, it can be difficult for the person to recognise that their "friend" is not real; at other times, they can be recognised as not being real, but are still used to offer comfort and company.
When I was three, I had one imaginary friend. When I was thirteen, I had five.